It has been almost 10 years since a CIBSE Journal CPD last discussed variants and applications of the ‘chilled beam’. In the meantime, the installation of beams has gathered pace, and they are now commonly seen in commercial applications. This CPD will revisit the basic functions of chilled beams, and explore the opportunities offered by the new generation of variable air volume (VAV) beams.

A prerequisite of maintaining safe and comfortable internal environments is good air quality, and this is typically achieved through ventilation. In commercial applications, mechanical ventilation will often also provide comfort cooling by supplying air at a lower temperature than room air.

When the outside, ambient air dry-bulb temperature is lower than the conditioned space dry-bulb temperature, this method is energy-efficient, as the outside air provides free cooling. Beyond the ventilative free cooling, moving cooling (and heating) energy around a building can be undertaken far more efficiently by pumping water (or other liquid) in pipes than by moving the same amount of heat in ducted air.

So, to increase cooling capacity and decouple air quality control from thermal comfort, ventilation systems are often combined with hydronic or variable refrigerant flow (VRF) systems, such as fan coil units (FCUs), radiant ceilings and chilled beams.

Chilled beams have coils that carry cool – or chilled – water and are mounted at high level in the conditioned space (principally in commercial buildings), and can be fully encased and suspended from the soffit or integrated into a false ceiling. The beams are typically referred to being either ‘passive’ or ‘active’ – as illustrated in Figure 1 and Figure 2.

Figure 1: Simplified passive chilled beam mounted above a perforated ceiling

The cool coil exchanges sensible heat with warm room air as the air passes across and between the cooler coils (and their extended surfaces). The temperature of the surfaces of the beam must not fall below the dew-point temperature of the surrounding air, otherwise condensation is likely. Any dehumidification (latent cooling) required in the space must be undertaken by an associated central air supply system.

Passive chilled beams rely on natural convection currents in the room, driven by denser cool air being drawn downwards by gravity and so displacing the less dense warmer air to a higher level where it meets the cool coils of the chilled beam.

Active chilled beams are supplied with ducted ‘primary’ air from a centralised system that will generally be used to meet the ventilation air requirement for the space. The primary air is supplied from the beam’s primary air plenum through small outlets (traditionally ‘nozzles’) directed towards the air diffuser outlet of the chilled beam casing. These fast-moving primary air jets create a high-velocity pressure.

Figure 2: Simplified active chilled beam suspended from soffit, integrated into false ceiling

The static pressure consequently reduces below that of the adjacent room air, so drawing in room air through the coil of the chilled beam (possibly via a ceiling void) and through the cool (or warm) coil, to be entrained into the stream of air supplied by the nozzles (as in Figure 2). This induction acts to significantly enhance the velocity of the room air – and so volume flowrate – passing across the coils, so improving the heat transfer to the room air.

The movement of the newly conditioned air across the conditioned space can be reinforced by the Coanda effect to ensure good diffusion distances (or ‘throw’) across the ceiling area. Active ‘chilled’ beams may be used as heating sources when supplied with low-temperature hot water (water temperature typically lower than 50°C to moderate air stratification) in place of cool water, while utilising warm primary air.

The primary air is often used to meet the latent loads in the space (that is, the dehumidification requirement) by having a supply air moisture content set at a level to offset the latent gains in the conditioned space.

To ensure dry cooling, which provides only sensible cooling to the indoor space via the hydronic circuit, the circulating water temperature is maintained above the room air dew point. Mitigating the risk of condensation-inducing humidity spikes in rooms involves employing sensors (such as that shown in Figure 3) on the cold-water supply that might trigger the closure of water valves when moisture levels rise.

Alternatively, continuous monitoring of room relative humidity can prompt thermostat-controlled closure of water valves nearing dew point conditions. Increasingly, condensation protection methods adjust supply water temperature based on room conditions, allowing for continued heat removal while minimising discomfort owing to reduced cooling capacity.

Figure 3: An example of a dew point (dp) temperature sensor that is fixed immediately upstream of the cooling. The dp is internally determined using a temperature-compensated relative humidity (RH) element and a high-accuracy thermistor, which are thermally bonded to the metal plate of the sensor. A volt-free contact relay activates when the dp temperature is below the set point (as set, in this case, using a screwdriver adjustment) (Source: Swegon)

Dry cooling systems rely on having sufficient air supply rates to manage latent loads. For high humidity environments, centralised air handling units (AHUs) must dehumidify outdoor air before distribution – this is often achieved through sorption-treated rotors. As discussed in more detail in REHVA’s chilled beam guide,¹ dry cooling offers several system advantages.

As with any hydronic system, moving water as a means of delivering cooling is far more effective than employing air and, in chilled beam applications, the moderately high chilled-water temperatures of 14°C to 18°C provide good opportunities for ambient free cooling. However, the central AHU typically requires chilled water at approximately 6°C to provide dehumidification.

The chiller plant may produce water at 6°C and the required 14°C to 18°C water can be delivered by mixing water from the chiller with return water, which is not an ideal solution; it has a deleterious effect on energy consumption (and system exergy) when compared with a conventional 6°C/12°C distributed chilled-water system, and the higher temperature of water being returned to the chiller plant will reduce its performance (energy efficiency ratio (EER)).

Energy consumption may be reduced if there are dedicated chilled-water production systems for the two different chilled-water temperatures – one for the centralised AHU system and one for the chilled beams. The chillers for the beams can work with a 14°C chilled-water temperature setpoint, which significantly increases their performance, lowers peak cooling demand and reduces the chiller size.

Free cooling can be implemented with, for example, cooling towers (for warmer/drier climates), dry coolers (for colder climates) and geothermal sources (water or ground). However, chilled beam systems will consume more pump energy in comparison with other distributed unit conditioners, as they employ a lower water temperature differential of 2K to 3K, compared with a conventional water temperature differential of 5K to 6K for traditional distributed unit conditioners. Where beams are used for heating, the lower hot-water temperatures of 32°C to 45°C are readily attainable from renewable sources.

Not only is the latent cooling centralised at the AHU, but it also provides driving pressure for room air distribution with central high-efficiency fans as opposed to local, smaller – likely lower efficiency – motor-driven fans that are used in other decentralised room conditioners.

Active chilled beams are engineered to operate without the presence of surface water, and typically eschew filters or drainage systems, so reducing service and maintenance needs –resulting in capital and operational cost savings compared with ‘wet’ systems. In such systems, filters would require periodic cleaning or replacement, and drainage systems demand maintenance to prevent microbial growth.

There are various hybrid beams, including the multi-service chilled beam (MSCB), that also integrate other services into the casing, such as ducts, lighting, cabling, audio equipment and sensors. They can provide an alternative to using additional trunking or possibly alleviate the need for a ceiling service void. They may be fitted directly to the soffit to potentially, for example, offer both a lighting and ventilation solution.

There are also hybrid systems that also incorporate a room-facing cooled surface to provide radiant heat exchange to the room in addition to the conductive heat exchange as the air passes through the coils and across the extended surfaces.

Traditionally, active chilled beams that supply air in constant air volume systems employ fixed geometry air paths that require manual adjustment, such as nozzle replacement, to significantly adjust the primary air flowrate. For a VAV setup, an air flow damper is typically mounted in front of the unit to adjust duct pressure upstream of the active chilled beam.

However, the relationship between duct pressure and airflow is not linear, as it follows a square law relationship so airflow (L.s^-1) q = k.√p_i where k is the flow coefficient and is related to the geometry of the air path, and p_i is the inlet pressure (Pa). When the damper reduces the primary air flowrates, the duct inlet pressure drops and the speed at which the air enters the room is consequently lower.

This will reduce the Coanda effect, so reducing the throw and increasing the likelihood of draughts. The induction ratio is practically constant, with a fixed value of k, so as the primary airflow is reduced, the amount of induced room air is reduced proportionally, as shown in the green line of Figure 4 – hence the heating/cooling capacity available from the water side is significantly reduced at lower primary airflows.

There are also limitations in increasing airflow. Boosting the airflow means increasing the duct pressure; however, the square law relationship can lead to unreasonable duct pressures that can generate increased noise levels. So, although traditional active chilled beams with fixed nozzles can be operated in VAV conditions, it will be within a limited range of primary airflow.

Figure 4: Induction ratio as a function of primary airflow for an example beam with variable geometry compared with fixed geometry outlets (Source: Swegon)

However, active chilled beams are now available where the traditional nozzles are replaced by adaptable geometry ‘slots’ in the supply air plenum. The amount of supply air delivered can then be altered by a controller actuating a change to the slot geometry to alter the value of k. Active chilled beams with variable geometry supplies can operate at different primary airflow rates while the inlet duct pressure remains constant.

As the primary airflow rate is reduced by decreasing the size of the openings in the pressure chamber, k reduces, resulting in an increased induction ratio as shown by the grey line of Figure 4. This enables high induction rates and good cooling/heating capacities even at low primary airflows.

The maintained duct pressure ensures that the speed of the supply air is kept at a level that can maintain the Coanda effect. Conversely, an increase in primary airflows does not require an increase in duct pressure, so a single unit can deliver a wide range of primary airflows.

When there is a need to change the temperature in the room, several parameters may be altered to adjust the amount of cooling/heating delivered to the room. The primary airflow and temperature, as well as the amount and temperature of water flowing through the coil of an active beam, can be adjusted, controlling the amount of heating/cooling delivered to a room.

If it is cold and dry outside, it might be advantageous to increase the airflow when there is a need to increase the amount of cooling. At another point in time, the conditions may be different, and it will be more efficient to increase the flow water rather than to increase the airflow.

Since active chilled beams rely on the induction of room air to generate a flow through the coil, the primary air is always mixed with room air that can be cooled, heated or, when the water valves are closed, unaffected by the coil. So, an active chilled beam might be considered as an induction diffuser, making it possible to operate with lower primary air temperature than required with most diffusers without facing issues with draughts.

A use case for this could be to deliver more cooling via the ventilation air or to reduce the primary airflow rates needed to deliver a moderate amount of cooling, which would make the use of free cooling even more efficient.

In the seemingly only recent independent, comprehensive (and reasonably priced) reference dedicated to chilled beams – the REHVA/ASHRAE Active and Passive Beam Application Design Guide¹ – it concludes that ‘compared to alternative HVAC systems, beam systems may offer significant savings in operating costs, namely in energy and maintenance. Replacement costs are also lower. …. The increase in the value of the building associated with the increase in the building useable area should be considered in the TCO (total cost of operation) analysis.’

With the advent of active chilled beams with variable values of k (‘VAV beams’), there may be further opportunities to apply this potentially energy-efficient hybrid solution that, if combined with a well-considered ‘smart’ control strategy, could help optimise the benefits of combined air and water system solutions.

© Tim Dwyer, 2024.

This article was inspired by some recent blogs by Carl-Ola Danielsson of Swegon.

What are the advantages of dry cooling?
Active chilled beams with variable k-factor – an option for both CAV and VAV systems

Our thanks to him for permitting the reuse of parts of his work in the article.

Further reading:

In addition to the REHVA/ASHRAE Active and Passive Beam Application Design Guide, a 2022 paper² by Latifa et al – Performance evaluation of active chilled beam systems for office buildings – A literature review – provides a good overview of many of the most recent published materials.

References:

1 Woollett, J, and Rimmer, J, Active and Passive Beam Application Design Guide, REHVA/ASHRAE 2016 – .

2 Latifa, H et al, Performance evaluation of active chilled beam systems for office buildings – A literature review, Sustainable Energy Technologies and Assessments issue 52, 2022.

The post Module 233: Variable geometry chilled beams for efficient variable air volume (VAV) systems appeared first on CIBSE Journal.

Effective life-cycle asset management, including performance audits and predictive maintenance of building services equipment, can make a significant contribution to decarbonisation efforts. This article will explore a key driver that has helped raise the level of industry expertise in this area, provide some examples of the elements and outcomes of engineering condition surveys, and reflect on the advance of digital technologies as a means of assessing systems’ performance – all as part of a holistic effort to decarbonise built environments.

A significant driver in the development of an increasing body of expertise for building services asset management has been the Energy Savings Opportunity Scheme (ESOS).¹ This is a mandatory energy assessment scheme for large UK undertakings that, although having its roots in the European Union (EU), is underpinned by UK legislation and will continue into the foreseeable post-Brexit future.

ESOS aims to identify tailored measures that lead to significant energy savings and carbon reductions, so organisations can choose actions based on their specific context and energy consumption patterns. The current ESOS phase 3 includes companies with more than 250 members of staff, a turnover greater than £44m and an annual balance sheet total of more than £38m.

ESOS assessments are conducted every four years to provide audits of the energy used by buildings, industrial processes and transport within the organisation. The ESOS reporting includes building energy intensity ratios (kWh per square metre) as well as total energy use in kWh or money spent. The rationale is that, while there are costs associated with the audit, the savings from implementing the recommendations typically outweigh the expenses.

ESOS mandates a comprehensive energy audit, which often requires improved data collection on building energy consumption. This might involve activities such as installing metering systems, monitoring equipment performance and analysing usage patterns.

ESOS can highlight maintenance issues impacting energy performance. For example, the audit might reveal the need for replacing faulty equipment, cleaning ventilation systems or fixing leaks in fluid distribution systems. ESOS compliance encourages a preventative maintenance approach that can identify potential problems before they escalate and lead to increased energy use. This proactive approach extends the lifespan of equipment and contributes to long-term energy savings.

ESOS can highlight the need for training building staff on energy-efficient practices. This might involve educating them on proper equipment operation, occupant behaviour that impacts energy use and procedures for reporting maintenance issues.

Developing and implementing a carbon reduction plan (CRP) is a central requirement for organisations complying with ESOS, in order to provide a roadmap for reducing their carbon footprint. By requiring organisations to assess their energy consumption, ESOS acts as a catalyst for cost-effective measures that improve building operation and maintenance practices.

The near future of ESOS looks to be focused on strengthening its impact on energy efficiency and aligning with the UK’s net zero targets. Phase 4 of ESOS – with a compliance period of 6 December 2023 to 5 December 2027 – will probably require assessments to consider not just energy savings, but also actions needed to achieve net zero emissions.

This might involve assessing the feasibility of renewable energy adoption or energy efficiency upgrades that contribute to carbon neutrality, while also identifying potential risks of moving towards net zero. ESOS 4 may introduce support to identify potential risks of moving to net zero alongside assistance in creating CRPs. Notably, the use of display energy certificates (DEC) and Green Deal Assessments will be removed as a route to ESOS compliance.

The UK government is considering mandating that organisations act on the recommendations identified in ESOS audits to ensure implementation, leading to genuine impact. Future requirements may demand that a greater range of ESOS data and results of subsequent actions are made publicly available, which could incentivise companies to take strong action for reputational reasons. Speculation suggests that there may be an alignment of ESOS with the Streamlined Energy and Carbon Reporting (SECR) scheme.² This could potentially bring smaller businesses under the ESOS umbrella. ESOS 4 is likely to see a further emphasis on data collection and analysis, as data-driven insights and automated controls can further optimise energy use and streamline the compliance process. This could include elements such as:

• Automated data collection and analysis
• Machine learning for optimising energy use within buildings
• Digital twin simulations for testing and evaluating energy-saving strategies.

Previously, there has not been a mandated auditing methodology; however, it is likely that BS ISO 50002³ and BS EN 16247-2⁴ energy audits – which set out a good-practice method for identifying energy savings opportunities – will be listed as the preferred route to reporting in ESOS 4.

Data-driven insight and digitally enabled automated systems are pervasive in the recent rewrites to CIBSE Guide M Maintenance engineering and management. This demonstrates the accelerating ubiquity of digital systems as the means of delivering effective intelligence on the operational state of building systems. Digitally sourced intelligence can combine with feedback from the building operator and performance management meetings to inform continuous improvement.

Deployment of digital technologies provides unprecedented opportunity for interaction with ‘smart’ sensors, building controls, maintenance history and predictive technology to measure and optimise performance. Automated reports or cloud-based dashboards are increasingly common as a means of monitoring systems that can be benchmarked against commissioning (and recommissioning) data to quantify performance and swiftly identify problems. This can provide the data for condition-based maintenance (CBM) that – unlike traditional planned preventative maintenance (PPM) – is not based on specific time intervals, but on regular, or continuous, monitoring of system performance to indicate any deterioration that may signal the need for recommissioning, maintenance, refurbishment and replacement procedures.

CBM can be employed to ensure resource utilisation is optimised, and prevent unnecessarily premature and inappropriate interventions. CBM may be undertaken completely onsite through, for example, the building management system (BMS), or facilitated remotely. Remote monitoring access may be as simple as an internet connection to the BMS, or it might employ continuously connected systems operated by the equipment manufacturer or the provider of maintenance support. Such services are increasingly being offered by equipment manufacturers using cloud-based, internet-connected systems that reference historical and contemporary data from the specific site, as well as the wider user base.

By employing techniques such as machine learning in conjunction with expert product knowledge, manufacturers can offer predictive performance assessment, preventative maintenance planning, and optimised refurbishment and replacement scheduling. The current challenge for the whole built environment industry is to federate this data, and to provide standardised secure, shared access to the streams of data that may usher in increasingly ‘intelligent’ buildings and cities.

Section M13 of the recently rewritten CIBSE Guide M describes the engineering condition survey (ECS) as being a process of systematically assessing the condition of assets in respect of their ability to perform their intended function. Traditionally, a manual (paper) approach has been used to collect data, and this remains an option for many small to medium installations by employing pro forma data-collection sheets.

However, this is typically being superseded by electronic data collection, through accessing BMS, service provider, and manufacturer-based systems and field records. Such surveys may be part of the regular management of assets, in support of the ESOS requirements, or they may be required to identify the extent of maintenance liabilities prior to taking on a property, or to inform a maintenance contract.

The ECS can provide an indication of the expected life of assets and components, which can be used to forecast the asset renewal demands and costs. Facility operators may vary the inspection cycle to suit the maintenance regime and assessment of risk to health and safety, operation and potential for failure.

Non-intrusive ‘visual’ inspection of the plant and associated systems and controlled spaces can provide an impression of the asset operation and wellbeing. However, this can inadvertently overlook unseen operational issues, as impending failure is not always visible. Valuable information can be obtained through conversations with maintenance staff and building users, as well as by reviewing logbooks, servicing reports and certification to help inform the overall view of condition.

While cost may prohibit frequent in-depth surveys, an annual visual inspection supplemented by a more thorough survey every three years might be considered as a reasonable compromise. A thorough survey should include examining maintenance reports for major plant items, and may require the attendance of specialists to provide further insight into the condition of assets.

Non-intrusive surveys can uncover failures that would not be picked up during a visual survey – CIBSE Guide M provides an extensive commentary on different non-intrusive techniques.

It is important that the assets surveyed are uniquely identifiable, with their type and location reliably recorded. A classification system for the major elements of the engineering services will need to be agreed at the outset of the survey. For example, this may employ a classification system from the Royal Institution of Chartered Surveyors’ (RICS) New Rules of Measurement,⁵ or systems such as Uniclass⁶ may be appropriate if, for example, asset data is already held in a building information model (BIM). Guide M suggests that categories may be applied to a survey report to help identify priorities for attention, ranging from priority 1 (urgent) to priority 4 (normal maintenance).

Much of the gathered information may be subjective, but is still necessary to give a ‘priority grading’ that allows the planner to prioritise the work to be carried out and model future budgets. Condition grades can provide an approach for recording the assessed condition of the assets to ensure a consistent interpretation by both the engineering surveyor and the recipient of the results. Guide M proposes a four-point condition scale, from A (fully operational) to D (very poor condition), with an extra designation of X if it is beyond repair.

The scope of an ECS will vary according to the requirements of the building and application. However, CIBSE Guide M considers that a typical survey report is likely to include detail on:

• Functional suitability of assets and components
• Operational integrity
• Physical condition
  
• Compliance with legal and health and safety requirements
  
• Conditions affecting operation and maintenance
• Economic life and obsolescence
• Energy efficiency of assets.

The format of the report will be determined by the client’s requirements, and include a summary for each asset identifying the maintenance and remedial requirements, their costs, and the timescale to undertake work. Spreadsheets may be used to illustrate expected maintenance and replacement costs based on predicted values (such as those illustrated in BSRIA guide BG 35/2012⁷).

This will then provide accessible and useful information enabling a client to set – with a reasonable level of confidence – priority objectives, and plan both short- and long-term maintenance and replacement needs. A comparable process was recently employed in the assessment and refurbishment of the chilled-water installation for a 20-year-old, 32-storey skyscraper office building in London’s Canary Wharf (see boxout).

The management of building services systems as a means of delivering safe, comfortable, effective built environments with reduced carbon footprints, is a truly holistic task. Starting with the original – or refit – design, through the appropriate selection and installation of systems (and their all-important controls) to eventual refurbishment and reuse, the effort to decarbonise will have the greatest opportunity to succeed only if the aspects discussed in this article are considered as part of the more extensive system that reaches far beyond the building façades.

References:

1. bit.ly/CJJun24CPD21 – accessed 2 May 2024.
2. Streamlined Energy and Carbon Reporting (SECR) scheme.
3. BS ISO 50002:2013 Energy audits, BSI 2013.
4. BS EN 16247-2:2022 Energy audits – Buildings, BSI 2022.
5. bit.ly/CJJun24CPD22 – accessed 2 May 2024.
6. Uniclass – uniclass.thenbs.com/ – accessed 2 May 2024.
7. Bell, R and Harris, J, BSRIA BG 35/2012 – Condition Surveys and Asset Data Capture, BSRIA 2012.

The post Module 234: Decarbonising commercial buildings through life-cycle asset management appeared first on CIBSE Journal.

Reliable methods are essential to assess the realistic operational energy performance of new homes, particularly when they include innovative applications of building services systems. By drawing on a recently reported study, this CPD illustrates the benefits of a properly matched energy system and how the misinterpretation of output from current regulatory tools can dissuade building purchasers and users from moving towards systems that can otherwise help move towards net zero homes.

Beyond passive measures, such as improving the thermal performance of the building envelope, the England Building Regulations AD Part L 2021¹ enhanced the requirements for mechanical components that contribute significantly to primary energy use and CO₂ emissions.

It can therefore be challenging to satisfy the present regulation (as determined by the UK’s standard assessment procedure² (SAP)) and achieve carbon reduction with the most advanced condensing conventional gas boilers. (The consultation has recently closed on the UK government’s draft home energy model (HEM), which will provide a more flexible steady-state calculation methodology designed to assess whether homes meet the Future Homes Standard; in 2025, it will replace the UK’s SAP. 

A recent study³ by Monodraught employed dynamic simulation modelling (DSM) for two example new-build, high-specification, four-bedroom homes to assess the energy and CO₂ performance for a novel ‘energy module’ package (using commercially available products) compared with a ‘conventional’ system. The conventional (baseline) system comprised a high-efficiency condensing gas boiler, hot-water cylinder and continuous operation extract fans.

The factory-tested packaged ‘energy module’ included an air source heat pump (ASHP), mechanical ventilation with heat recovery (MVHR), and a hot-water thermal battery, all as discussed in the panel ‘Packaged “energy module”’). The two services variants for each of the two homes – detached house type A (Figure 1) and semi-detached house type B (Figure 2) – were also evaluated using the UK government’s SAP calculations.

Figure 1: Detached house type A floor plans (ground floor, first floor) – image courtesy of Woodall Homes

Figure 2: Semi-detached house type B floor plans (ground floor, first floor, mansard) – image courtesy of Woodall Homes

The fabric U values in the homes were at least as good as – and most improved upon – Part L limit values.

In the baseline case, outdoor ventilation air is drawn through ventilation slots incorporated in the windows of bedrooms and living rooms. The bathrooms, the utility room and kitchen are fitted with decentralised mechanical extract ventilation (dMEV) fans with two-speed control that meet the AD F boost ventilation requirements. The specific fan power (SFP) of extract fans was taken as 0.3 for kitchens and 0.25 for other wet areas. With the ‘energy module’ MVHR, air is supplied to each bedroom and living room, and extracted at bathroom, toilet, utility room and kitchen areas.

Using the guidelines in England Building Regulations AD Part F,⁴ the required supply and extract ventilation rates were calculated for each space. For house type A, the total ‘high rate’ for extract ventilation exceeds the required minimal supply, so background ventilation was set to 37L.s^-1 and the boost rate of 43L.s^-1 during the active occupied hours in the kitchen and bathrooms. House type B can always operate with 37L.s^-1, which meets both the minimum total ventilation rate for the dwelling and the boost extract rates of the wet areas.

The houses have an airtightness of4.0m³.h^-1.m^-2@ 50Pa that translates to a total of 10.5L.s^-1 and 6.7L.s^-1 for house type A and house type B respectively. Although the minimum ventilation is provided by mechanical means, temperatures may be controlled by opening the windows.

The modelling of natural ventilation was set so windows were activated whenever room temperatures exceeded 23°C, to avoid overheating, although CIBSE TM59⁵ (assessing overheating risk) and England Building Regulations AD Part O⁶ (mitigation of overheating) were not part of this study.

Occupancy and equipment gains were distributed over 24 hours according to profiles detailed in CIBSE TM59.⁵ Internal lighting efficiency was set to 80Lm.W^-1 in accordance with AD Part L1 2021,¹ and standard lighting levels were applied according to occupancy type. Lighting was simulated to operate based on photosensors whenever the natural light decreased below the desired level within each occupied room, except bathrooms where the lighting is controlled according to a fixed occupancy schedule at the start and end of the day.

The baseline configuration of the houses includes gas-fired condensing boilers with a seasonal coefficient of performance (SCOP) of 93%, or seasonal efficiency of domestic boilers in the UK (SEDBUK) rating of 89.4% for heating and domestic hot water (DHW) employing a 180L hot-water cylinder with a standing loss of 67W at 60°C.

Based on CIBSE Guide G,⁷ the daily hot-water demand was calculated as 115L per person at 65°C for six occupants, which equates to a modelled 31.15L.h^-1 hot-water consumption at 60°C. The baseline DHW power demand was 1.81kW. Adding the standing loss and a 0.93 boiler efficiency, 2.02kW will be needed for hot-water production – an annual demand of 17.67MWh.

The phase change material (PCM) battery delivers DHW at 40°C without water storage and no risk of legionella to the consumer, and is targeted at an end use of 40°C. The hot-water flowrate was increased to 46.72L.h^-1 @ 40°C, which is equivalent to 31.15L.h^-1 @ 60 °C. The energy necessary to heat up 46.72L.h^-1 mains water from 10°C to 40°C is 1.628kW, slightly lower than the baseline system.

Taking account of storage losses, 0.22kW of power can be potentially saved with the PCM battery, which means a 1.58MWh annual saving in hot-water production with the new technology. 

CO₂ emission and primary energy factors for electricity were based on the latest figures of the National Calculation Methodology (NCM) guideline , which varies slightly in each month. For natural gas, typical figures were employed of
0.21kgCO₂.kWh^-1 and 1.126kWhPE_PE.kWh^-1.

The annual primary energy figures for the studied scenarios are summarised in Figure 3. The DHW energy is the driving factor in the annual energy consumption, comprising 90% of the whole system energy compared with only 4% energy spent for space heating and 6% for fan/pump energy. The heating energy and all other energy categories are relatively small compared with the DHW consumption.

The envelope heat losses are small for homes designed for net zero so, in energy simulation, heat generated by equipment offsets much of the fabric and infiltration heat loss. When swapping the gas boiler with an ASHP the heating energy at house type A and house type B decreased by 91% and 64% respectively.

Changing the location of the homes to Southampton (which has milder weather) did not significantly impact the overall system energy. Although the space-heating demand dropped by 36%, there was just a 1.4% drop in the overall energy consumption in the ‘energy module’ applications, as 91% of energy was attributed to DHW.

The heating and ventilating energy reduction in both house types resulting from applying the ‘energy module’ was 80-82% lower than the baseline case. When including the unchanged equipment and lighting energy requirement, the overall energy saving came to 74% in both house types. In terms of carbon savings calculated for systems and lighting, both house types emitted 82% lower CO₂ for the ‘energy module’ system compared with baseline.

The main indicators of the SAP calculations in Figure 4 show the CO₂ emissions and primary energy for the actual dwellings (dwelling emission rate (DER) and dwelling primary energy rate (DPER)) were significantly reduced in case of the ‘energy module’, while the dwelling fabric energy efficiency rate (DFEE) remains the same.

The DPER reduced by 52% and DER CO₂ emissions by 75% when applying the ‘energy module’. The SAP environmental impact (EI) rating went from Category B to Category A when the buildings were served by the ‘energy module’.

Despite the significant energy and CO₂ performance improvements of the homes that employed an ‘energy module’, it did not improve the SAP rating, as that is contingent on the notional running costs of a home. The dynamic model showed that the system energy for the ‘energy module’ is a third of the baseline case.

However, the SAP fixed cost for electricity is 4.53 times higher than gas, so an ASHP system is unable to demonstrate an improved SAP rating. (The current UK residential price cap sets electricity at 3.85-times the cost of gas.) As the SAP rating is a key indicator for property purchasers and landlords, it will remain challenging to encourage the investment needed in ‘alternative’ technologies if future homes decisions are made on a regulation compliance rating dominated by nominal energy prices, rather than primary energy and the increasingly pressing considerations of the environment.

References:

1. England Building Regulations Conservation of fuel and power Approved Document Part L Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, bit.ly/CJMay24CPD1 – accessed 2 April 2024.
   
2. bit.ly/CJMay24CPD2 – accessed 2 April 2024.
   
3. Bakó-Biró, Z and Hopper, N, HomeZero performance in DSM and SAP modelling for domestic properties,
4. Monodraught 2024.
   
5. England Building Regulations Ventilation Approved Document F Volume 1 applies to dwellings, 2021, bit.ly/CJMay24CPD1 – accessed 2 April 2024.
   
6. CIBSE TM59 Design methodology for the assessment of overheating risk in homes, CIBSE 2017.
   
7. England Building Regulations Overheating Approved Document O, 2022, bit.ly/CJMay24CPD1 – accessed 2 April 2024.
   
8. CIBSE Guide G Public health and plumbing engineering, CIBSE 2014. bit.ly/CJMay24CPD3 – accessed 2 April 2024.

The post Module 232: Assessing the realistic performance of new homes and systems appeared first on CIBSE Journal.

The global drive to reduce building-related emissions and the ambition to achieve net zero by 2050 requires a significant shift in how all buildings are heated and cooled. Decarbonising heating in non-domestic buildings – which account for a large proportion of emissions – will make a significant contribution towards reaching this goal. This CPD article, which draws extensively on elements of the freely-downloadable CIBSE applications manual AM17, explores the early-stage investigations and assessments that will set non-domestic building systems designers, owners and operators on a solid foundation for the development of a heat pump application.

To meet the existing energy and climate pledges worldwide, the International Energy Agency (IEA) reports¹ that heat pumps will have to meet nearly 20% of global heating needs in buildings by 2030. IEA considers that the world is almost on track to reach this milestone if new installations continue to grow at a similar rate globally of more than 15% per year, as has been achieved in recent years.

In stark contrast, the recent update² by the UK government’s Heat Pump Ready initiative – which is, at least currently, focused on accelerating the number of domestic heat pump installations – reported that the UK required an annual compound market growth of 43% per year to deliver the UK government target of 600,000 heat pump installations annually by 2028. (UK growth was reported as being far lower than that in several European states with similar climates.) In its recent report,¹ IEA notes that installations of heat pumps remain concentrated in new buildings and existing single-family homes, and that multi-storey apartment buildings and commercial spaces will need to be a priority area if solid growth is to continue.

It is thought that the majority of the heat pumps installed in the UK in 2023 were for domestic applications.² However, (non-electrical) heating and hot water utilise³ approximately 40% of the energy supplied to UK non-domestic premises, and therefore potentially provide a great opportunity for heat pump applications.

Heat pump technology continues to advance, most recently with the increasing rollout of ‘high temperature’ heat pumps – which typically employ environmentally benign refrigerants such as CO₂ and propane. This eases the transition for existing buildings, as well as new developments, to employ heat pumps as the main – or even acting as the sole – heat generator for non-domestic applications.

CIBSE AM17 Heat pumps for large non-domestic buildings serves as a comprehensive guide to support the effective design, installation, commissioning, operation and maintenance of heat pump systems of more than 45kW (see boxout overleaf). AM17 provides a technical commentary encompassing all the assessment and implementation stages of heat pump deployment, as illustrated in Figure 1.

AM17 emphasises the importance of minimising heating and cooling demands before designing a heat pump system. Smaller, more efficient and more affordable heat pumps become possible by reducing peak loads. Ideally, this would include optimising the building form, which is clearly more achievable in new developments.

However, the thermal performance of existing buildings can equally be enhanced by modifications such as improvements in building fabric, greater airtightness, application of thermal mass and appropriate solar shading. Demand-reduction strategies should be holistic, considering both heating and cooling demands throughout the year, while indoor environmental quality and occupant comfort must, of course, remain crucial considerations when attempting to reduce demands. The total embodied carbon of building improvements must be balanced against the reduced carbon footprint of a smaller heat pump system.

The optimisation of building thermal performance not only benefits the building owner, but also reduces the strain on the electricity grid and the knock-on impact on surrounding developments. It is recognised that not all buildings, especially those with heritage constraints, can implement every demand-reduction measure, but design teams should consider such strategies wherever possible. The process of optimisation is likely to be iterative, as various packages of interdependent building and system parameters will need to be explored to provide the best solution for a particular application.

The appropriate sizing and selection of heat pumps – such as the monobloc unit illustrated in Figure 2 – that can meet the heating demands of the building is key to avoid shortcomings in terms of capital and operational costs, space, noise, efficiency and life-cycle emissions. The fundamental requirement is to examine the loads and temporal zonal load profiles, considering intended use, building usage patterns, plus factors including fresh air ventilation rates, internal design conditions and any process loads. AM17 notes that standard methods of load estimation are provided in CIBSE Guide A⁴ and BS EN 12831-1:2017.⁵

Dynamic thermal modelling and simulations (which are increasingly enabled through a data-rich BIM model) can provide useful intelligence around loads and time-dependent profiles that inform scenario and sensitivity analysis to help understand the potential impact of varying building parameters. This could include such areas as building construction details; operational setpoints; optimised, adaptive ventilation methods; and the integration of thermal storage.

Careful use of adaptive and weather-compensated internal design conditions can provide significant load reductions (as long as the system finally installed is appropriately controlled). Any analysis should reflect impending climate scenarios by employing appropriate future weather files that account for potential shifts in temperature and precipitation. For existing buildings, co-heating tests, metered data and tenant information may be used to aid the prediction of future loads. Annual, daily, and hourly profiles can reveal periods of simultaneous heating and cooling, as a means of establishing an understanding of peak instantaneous load. CIBSE TM54⁶ provides methodologies for the estimation of the annual energy consumption in buildings.

When selecting and sizing the central heat pump system, diversity factors for zonal and occupational demands are likely to have a more significant impact on oversizing than with traditional fossil-fuel systems. It is important to take a measured approach to the impact of future tenant loads, as being over-generous with future allowances could lead to an oversized and probably less efficient system. Domestic hot water demands are notably challenging to predict, and the demand should be diversified according to an appropriate standard for the application. For a discussion of the issues associated with this, see CIBSE Technical Bulletin No 1.⁷

Non-commercial buildings that have high domestic hot water use – for example, hotels, multi-residential developments, restaurants, healthcare facilities and leisure centres – require particular care, as assumptions made at the early stages of design can be overturned with later system selections that can significantly impact the previously expected performance of the building thermal systems. Systems should be planned for variations in demand as different parts of a development are occupied, as well as to cater for future building needs.

Understanding how often the system is likely to perform at part-load and mid-load will help inform designers and operators how the systems are likely to operate prior to full building occupation or during periods of unpredictable and fluctuating usage patterns. Each type of building, and each application, will have specific needs. AM17 provides a high-level summary of some of considerations required for a selection of building types, shown in Table 1, that illustrates the need for careful consideration of specific applications.

Systems are typically designed with some resilience that permits operation while, for example, individual heat generators (and associated sub-systems) are maintained or repaired; when buildings experience surge heating or cooling loads; or simply to accommodate a need to reduce building heat-up times. Careful analysis is required to establish the optimum arrangement of heat pump units and thermal storage to deliver the most effective solution.

Thermal storage is increasingly employed to maximise the heat exchange between cold and hot water generation, which can significantly improve operational efficiency. This can be particularly effective where there are coincident heating and cooling demands, such as domestic hot water on a summer day when space cooling is required; where different zones of the building have disparate heating and cooling demands; or possibly where there is an opportunity to balance heating or cooling process loads. Temporal storage can be an asset, particularly in areas with mild climates, allowing time-shifting of loads to provide greatest overall efficiencies by storing either high- or low-temperature heat.

Any analysis of the requirements for thermal storage should be based on hourly modelling and include the impact stores have on overall heat losses and heat sharing capacity. Heat pump effectiveness will deteriorate with excessive on-off cycling, as clearly demonstrated for domestic applications in the 2012 report by Robert Green.⁸ This can be significantly reduced through having sufficient thermal capacity in the water distribution system and associated thermal stores. In commercial systems employing multiple, properly sized and staged heat pumps, excessive cycling should not be a significant problem.

If there is a desire, or a need, to employ bivalent or multivalent systems – where loads are shared with, for example, fossil fuel boilers, direct electrical heaters, water chillers and solar thermal collectors – then an assessment must be undertaken to determine the proportion of the loads that will be provided by each technology. This may be relatively simple if, for example, a gas boiler is providing top-up heating for a low-temperature heat pump to ensure legionella-safe domestic hot water, but, otherwise, further iterative analysis will be needed that will include assumptions of the future system selection and design.

Bivalent solutions have been relatively common with low-temperature heat pumps (where higher temperatures have been required to meet the loads); however, as heat pumps evolve to effectively provide higher flow temperatures, this is likely to become less of a technical issue. Lower hot water and higher chilled water temperatures will tend to improve heat pump performance, as well as reduce distribution thermal losses. In turn, these will also impact components and their performance – including pipe sizes, pumps, heat emitters and heat exchangers – so will add a further level of complexity to the optimisation process.

A well-designed and installed heat pump system can provide significant cost savings compared with traditional heating and cooling systems. However, proper system design, installation, commissioning, operation and maintenance are crucial to ensure optimal performance, energy efficiency and longevity of heat pump systems. Without an appropriate foundation of assessment and analysis, it is unlikely that the installed solution will provide the desired – and essential – decarbonisation that can otherwise be delivered by heat pump applications.

References:

1. bit.ly/CJApr24CPD2 – accessed 1 March 2024.
2. bit.ly/CJApr24CPD3 – accessed 10 March 2024.
3. Building Energy Efficiency Survey (BEES), UK BEIS, 2016.
4. CIBSE Guide A.
5. BS EN 12831-1:2017: Energy performance of buildings. Method for calculation of the design heat load, BSI 2017.
6. CIBSE TM54: Evaluating operational energy use at the design stage, CIBSE 2022.
7. CIBSE Technical Bulletin 01: Domestic water demand assessment for pipe sizing.
8. Green, R, The effects of cycling on heat pump performance, UK DECC, 2012.

The post Module 231: Foundations for successful heat pump installations in non-domestic buildings appeared first on CIBSE Journal.

This article outlines the factors that drive the need for water treatment in closed-loop water systems and focuses on the processes that provide ‘chemical-free’ water treatments. It also explores how such systems provide treatment of closed-loop water systems, such as those used in heating and cooling systems for buildings.

Water treatment is typically undertaken at the pre-commissioning stage and as an ongoing process to establish and maintain water quality. This ensures that the circulating water does not become contaminated with particulate matter, bacteria or other harmful substances (such as shown in Figure 1), which can lead to system failure.

As discussed in the comprehensive guide BSRIA BG 50,¹ appropriate water treatment can help to inhibit corrosion and protect the metal components of the system, so reducing the opportunity for leaks, reductions in efficiency and, ultimately, system failure. It can prevent the formation of scale on the inside of pipes, fittings and heat exchangers that will otherwise reduce the efficiency of the system and reduce system reliability. Appropriate water treatment can also control microbial growth, helping to prevent bacteria and other microorganisms that can grow in closed systems, causing problems such as corrosion and fouling. 

Monitoring and controlling the water quality across the life of the installation will improve system performance, lower energy costs, reduce the need for repairs and maintenance, and potentially extend the life of the system.

There are several parameters that will impact the water quality in closed-loop systems. Oxygen dissolved in the water acts as a powerful oxidising agent, readily interacting with metals and accelerating corrosion processes. Dissolved oxygen may be minimised through proper system design, degasification, and by maintaining a closed loop with minimal air ingress.

Total dissolved solids (TDS) – an amalgam of minerals, ions, and other dissolved substances in the water – will increase the water’s electrical conductivity and impact the rate of scale accumulation and corrosion. Water ‘hardness’ is specifically identified as the amount of calcium and magnesium salts in water, generally in the form of bicarbonates, chlorides and sulphates that, if left, will accumulate as insoluble carbonate (scale) in water systems. Chloride ions are notorious for promoting localised corrosion, as they can penetrate and destabilise the naturally forming protective oxide layer, leaving the underlying metal vulnerable to corrosion.

The acidity or alkalinity of the water, as measured by the pH, significantly influences corrosion rates. Generally, lower pH (more acidic) environments increase corrosion, as this dissolves the protective oxide layer on the surface of metals, leaving the underlying metal exposed and vulnerable to corrosion.

Figure 1: A 40μm filter basket from a reaction tank (as in Figure 3) that was removed shortly after installation into a new-build system

Conversely, slightly alkaline environments can help thicken the protective oxide layer, offering better protection – although excessively high pH can induce other degradation processes (such as disrupting the passivity of stainless-steel components).

Higher pH will tend to reduce most bacterial growth. However, the bacteriological quality of the water will be highly dependent on the cleanliness of the initial system and the fill water.

These factors interact and influence each other, so a holistic approach – considering all relevant parameters and implementing a customised treatment plan based on a specific system and water characteristics – is key to minimising corrosion risk and ensuring the longevity of a closed-loop system.

There are several methods that are used to treat water for closed-loop systems either for continuous use, or to treat fill or make-up water, that may be applied individually or in conjunction with others, depending on the specific system requirements. Physical methods are often used to remove impurities from the water.

This may be through some form of filtration to remove particulate matter, by deaeration methods that reduce dissolved and partially dissolved gases; and employing techniques such as reverse osmosis, which forces water through a semipermeable membrane that has microscopic pores to allow water molecules – but not most contaminants – to pass through.

Resin-based demineralisation units (or cartridges), strainers, softeners and clarifiers are employed to remove impurities from the water. 

In the UK, chemical water treatment is currently the most common type of water treatment for closed systems, where chemicals are added to the water to inhibit corrosion, prevent scale formation, and kill bacteria. However, chemical-free is becoming increasingly popular – a method of water treatment for closed-loop hydronic systems that does not use inhibitors or biocides.

The method employs an initial fill of demineralised, clean water, and then electrochemistry is used to control the causal elements of scale and corrosion – minerals, salts, oxygen and other gasses. In recent years, this method has increased in popularity; it also meets the requirements of Germany’s influential guideline VDI 2035² .

Chemical-free water treatment systems typically consist of a demineralisation unit and a reaction tank, and although this employs chemical and electrochemical processes, it does not add any chemicals to the system water.

As shown in the example system in Figure 2, mains water passes through the demineralisation tank, which contains a mix of cation and anion exchange resins. Cation resins exchange positive ions such as calcium and magnesium ions with sodium ions, and anion resins exchange negative ions and remove dissolved solids, salts and other ions from water, so that the fill water has no, or low, conductivity. The mixed resin also removes carbon dioxide.

Over time, the resin becomes saturated with contaminants, and this will lead to decreasing water quality. Resin capacity is dependent on the hardness of the local water and the total flow through the unit over time. As the resin becomes saturated with mineral ions it needs regeneration or replacement. Regular monitoring of conductivity or specific ion levels helps determine when resins can no longer perform their function. Resin typically has a useful life of three years in a system, when it will then require changing even if it has not become saturated.

The reaction tank (such as that shown in Figure 3) employs sacrificial magnesium anodes surrounded by a filter and a strainer, which is then contained in a cylindrical stainless-steel housing. The tank is positioned in the system where the water is hottest (for example, near the boiler output – as in Figure 2 – or the input to a chiller), as this is where there will be the greatest number of bubbles of entrained air and gases. 

In the illustrated reaction tank, the water enters tangentially, creating a swirling motion, and as a result of centrifugal forces, heavier particulate matter is thrown outwards, which then falls and collects at the base of the unit. The outer cylindrical strainer captures any remaining larger particles while a finer (micron) inner filter removes smaller particles. These particles will be held in the strainer and micron filter, and will drop to the lower chamber when the unit is being ‘blown down’. (During scheduled maintenance, the blow-down valve is briefly opened to force a controlled flow of water back through the filter and strainer screen, in order to force out accumulated debris through the blow-down valve into an appropriate waste-disposal vessel. The valve is then closed.)

This filter also traps micro air and gas bubbles, which then amalgamate and buoyantly rise, to be removed from the system by an automatic air vent. This degassing process is continuous, and when the treated, degassed water leaves the tank, it cools and passes around the distribution system absorbing trapped system air that will, in turn, be removed to atmosphere as the water passes back through the reaction tank.

The magnesium anodes in the tank have significantly more negative electrochemical potential and so are more ‘active’, so they corrode when they are in electrical contact with the stainless-steel outer shell and stainless-steel filter assembly – which act as a cathode – in the presence of an electrolyte (such as the ion containing water).

Reaction key:

1 Automatic air vent 2 Magnet to capture metallic particles 3 Air and other gases rise through buoyancy forces 4 Water enters with entrained particles, dissolved oxygen and air micro bubbles (located at point of hottest system water) 5 Magnesium anodes 6 Strainer to capture larger particles. 7 Approximate water path 8 Micron filter (40μm) basket to remove smaller particles and entrained air – can be swapped for bag filters down to 0.5μm for finer particle filtration 9 Dirt, sludge, debris, and metallic particles (including the magnesium residue from the expiring anodes) collect inside the basket 10 Blow-down valve periodically used to remove accumulated matter

As a result of the electrochemical reaction between the magnesium anodes and the surrounding cathode, magnesium hydroxide is produced (at the anode), and any dissolved oxygen is gradually removed (through a reaction at the stainless-steel) as the water recirculates. Hydroxide ions produced at the cathode increase the pH of the water – a process known as ‘self-alkalisation’.

However, excessive alkalinity can also lead to issues such as scale formation and reduced heat-transfer efficiency, so the pH should be carefully monitored. (The electrochemical reactions also produce chloride and hydrogen gas, which rises to the top of the tank to be released to the atmosphere.) 

The cathode and the anodes are connected through a galvanometer that shows the electrical flow between the two. As pure water is non-conducting, more impurities and oxygen in the water will increase the current. As the water quality improves, the current diminishes. The system is self-regulating, as the anode automatically works harder with corrosive water. All the components should be cleaned on a regular maintenance cycle, which will be scheduled depending on the installation.

Reaction tanks are typically sized based on the system heating or cooling load, the system volume, and the system operating temperature. So, for example, chilled water and heat pump systems will typically need a larger unit than a boiler-fed system, as they do not benefit from the higher temperatures to assist the removal of bacteria – this is achieved by a greater surface area of the anodes.

For the initial system fill, a chemical-free installation requires ideal fill water that is bacteria-free with a controlled pH and low conductivity. Typically, the fill will pass through a bacteria filter of 0.5 microns or a reverse osmosis filter (with an appropriate protective pre-filter) to block microbes from entering the system.

The water will then pass through a mixed demineralising resin bed to deliver the required quality fill water. Early operation of the system at its maximum temperature after filling should be carried out to remove gas and air pockets. Once filled, the installation debris is cleared by circulating the system water to velocities as recommended in BSRIA BG 29,³ using a mobile high-flow filtration unit that filters the system water without discarding any water to drain. 

As reflected in VDI 2035,² BSRIA BG50,¹ and CIBSE Guide M,⁴ system performance monitoring is essential for the proper maintenance of closed heating and cooling systems. It can help to identify potential problems early on, prevent costly repairs, and ensure that the system is operating efficiently.

Recommendations in CIBSE CP1⁵ are to continually monitor corrosion in the system water using electronic coupons (that mimic the behaviour of a physical metal coupon to measure corrosion parameters electronically), which, in conjunction with automatic pH and TDS monitoring systems, can feed into building management systems (BMS). Reaction tanks are available with real-time monitoring that can deliver information both locally and directly to a BMS to provide a continuous record of these critical factors. 

Despite being widely embraced for many years across Europe, it is only in recent years that chemical-free water treatment for closed-loop systems has been more widely adopted in the UK. It is increasingly being referenced by HVAC manufacturers as being suitable for systems operation, and is being adopted by a growing number of designers and users.

Further reading:

Chapter 12 of CIBSE Guide G provides an excellent foundation for understanding corrosion and corrosion protection.

Annex E of CIBSE CP1 Heat networks: Code of Practice for the UK (2020) provides useful tables of recommended parameter limits for heat networks (closed-loop networks).

References:

1. Simpson, P, BSRIA BG 50/2021: Water treatment for closed heating and cooling systems’, BSRIA 2021.
   
2. VDI 2035 – Part 1 : Prevention of damage in water heating installations – Scale formation and waterside corrosion, Verein Deutscher Ingenieure 2021.
3. Parsloe, C, and Ronceray, M, BSRIA BG 29/2021 Pre-commission cleaning of pipework systems, BSRIA 2021.
4. CIBSE Guide M.
5. CIBSE CP1: Heat networks: Code of Practice for the UK, CIBSE 2020.

The post Module 230: Chemical-free water treatment appeared first on CIBSE Journal.

The field of risk management, including risk assessment and mitigation, has seen significant growth in the past 30 years, highlighting the increasing importance of managing risks effectively across various contexts. Drawing on the material that contributes to the new version of CIBSE AM4.1 Security engineering: Strategy, this article will introduce security risk management processes for building services engineering applications. 

Security risk management goes hand-in-hand with business continuity and resilience, and each benefits from the active input of dedicated professionals. Designers of built environments must understand appropriate security concepts so that they can identify requirements and technologies in order to integrate them into their designs for a specific project.

Built environments can have a variety of potential security risks that may be influenced by actions, designs, operations, and processes that are associated with the activity of the building services engineer.

These may include a diverse range of areas such as: perimeter and internal security; access control; theft prevention; vandalism control; terrorism and natural disaster; threats to the person; fire safety; environmental and biological safety; occupant health and safety; information, document, IT and cyber security.

Once threats, mitigation options and a strategy are defined, security engineering comes into play. This focuses on designing, specifying and integrating physical, technical, and procedural security measures. Prior to attempting to develop designs for security measures, it is essential to properly define what needs protection by identifying valuable assets in the built environment, and how these may be threatened.

A holistic – and likely cyclic – approach will identify threats, which will help to achieve desired security outcomes, ultimately reducing vulnerability and risk, as illustrated in the simplified example risk management cycle of Figure 1.

Security engineering contributes just one aspect of the procedure and resides towards the end of the security risk management process, relying on prior risk assessments, prioritisation and decision-making.

Building services engineers, while experts in making buildings function, typically lack the specialised skills required to design and specify technical security systems within a defined security strategy, and so collaboration with security professionals provides a valuable, if not essential, pathway to a holistic risk assessment, management and mitigation process.

Security in the built environment should be a structured and transparent process, with solutions tailored to the specific risks and needs of each project, and it is unlikely to be satisfied with generalised solutions. It must be planned and designed collaboratively with other disciplines such as architecture, civil and structural engineering, and landscape architecture.

While traditional threat assessments focus on motive and capacity (intent and capability), a comprehensive understanding demands broader examination. Group dynamics, past activities, ideological motivations, preferred attack methods, and the wider security landscape all play crucial roles. Different threats and tactics necessitate a tiered system for categorising their severity, ensuring clarity and precision. Table 1 provides an example of this gradation.

Vulnerability assessments quantify the likelihood of assets succumbing to an attack. They evaluate the effectiveness of potential measures (deter, detect, delay/deny, respond, recover (DDDRR)) and ensure no weak links compromise the entire system. Similarly to the graded threat levels, a vulnerability rating system can be established defining categories from very low to very high vulnerability.

The effective risk assessment process goes beyond just the ‘who’ and ‘how’ of potential threats; it delves into the likelihood of a threat materialising and the resulting consequences. By understanding these two factors, risks may be effectively prioritised and appropriate resources allocated.

Traditionally, some consider ‘likelihood’ solely as a function of threat and vulnerability levels. While this may work in certain scenarios, it may overlook crucial factors such as asset criticality and target attractiveness. A highly desirable target under high threat with significant vulnerabilities will naturally have a higher chance of attack.

‘Consequence’ is the overall impact of a security event, encompassing areas such as human harm, financial loss, reputational damage and business continuity disruption. While these are common areas of analysis, other specific impacts may be relevant, depending on the project. The combination of likelihood and consequence determines the overall rating of a risk event, which is often visualised through a risk assessment matrix, such as the example in Figure 2, providing a clear basis for stakeholders to evaluate and prioritise risks.

While attempts exist to quantify risk through numerical values, these should be approached with caution. Security risk assessments are inherently qualitative, and assigning arbitrary numbers can be misleading. Once assessed, risks must be prioritised for management or mitigation.

This crucial step aims to identify which risks require active intervention and which can be accepted or tolerated. For example, ‘very high’ and ‘high’ risks may be prioritised for management to reduce both likelihood and consequence. Conversely, low-impact, low-likelihood risks can be accepted with minimal monitoring.

However, other scenarios require more nuanced decision-making, such as risks with low likelihood but catastrophic consequences, or high likelihood but lower consequences. Ultimately, ‘very high’ and ‘high’ risks should inform the development of ‘most-credible, worst-case scenarios’ (MCWCS), which guide risk management actions.

The prioritisation of risks and MCWCS should be formally documented in a project security brief to ensure awareness across stakeholders. Typical key outputs that built environment security risk management professionals deliver as part of the development of a security brief are shown in Table 2.

The risk assessment process would typically require the input provided by security consultants and security engineers. Security risk consultants focus on the big picture – assessing risks, developing comprehensive security strategies, and integrating physical, technical, and operational measures. They are the architects of the overall security approach, playing a leading role early on in planning and design, establishing the foundation and overall security strategy while ensuring harmony with other project goals. They may also offer input on new technologies.

During construction, the consultant takes a light, oversight role to ensure the designed security strategy stays on track; post-construction, they become more involved, participating in security reviews, audits, and oversight activities to guarantee ongoing risk management.

Security engineers focus on the specifics – designing, implementing, and maintaining the technical and physical security solutions defined in the strategy. They are the builders and implementers of the security plan, and are primarily involved later in the detailed design and technical stages. They focus on designing, installing, and commissioning security equipment.

In essence, consultants analyse and plan throughout the project life-cycle, while engineers execute the technical details through the design and build. Both are crucial for effective security, but their skills and contributions differ throughout the risk management process.

To meet the demands of each individual project, the risk management process sets out a series of steps for tackling potential issues. It starts with pinpointing risks, followed by in-depth analysis, prioritisation, solution implementation and ongoing monitoring. These steps, often laden with paperwork and administrative tasks, have paved the way for software-assisted frameworks.

Many frameworks and standards have been developed to guide risk management practices, most notably BS ISO 31000:2018,¹ which provide standardised approaches and best practices for identifying, analysing, and controlling risks. The presence of multiple frameworks might be reflective of the complex and nuanced nature of risk management, each offering tailored approaches depending on the specific context and needs.

There are several common routes to integrate security into a project. The most suitable route will depend on the specific security needs of the project. 

A security-needs assessment (SNA) may be employed to identify site-specific security risks and vulnerabilities that would involve consultation with stakeholders, including the police. A successful SNA (including implementing recommendations) may be used to achieve Breeam ‘HEA 06 – Security’ exemplary level credit.² The Breeam guidance² provides a useful definition for what is considered as a ‘Suitably Qualified Security Specialist’ (SQSS) for the purposes of such work.

Secured by design (SBD)³ focuses on incorporating security measures into properties by accrediting security products and developments. This is operated by the UK Police Services as their preferred scheme for demonstrating how security has been integrated into a new development to deter criminal and anti-social behaviour through the design, layout and specification of buildings and the spaces around and between them. It may be used to meet specific area planning conditions. 

SABRE⁴ is jointly operated by The Security Institute and BRE and is aimed at reflecting best practice in security risk management by emphasising security that is appropriate, proportionate and fit for purpose. It establishes the required documentation to evidence security decision-making. SABRE certification is a reflection of the security risk management process and documentation of security decision-making on a project and, in itself, does not specifically indicate that a development is more secure. Breeam guidance² defines what constitutes a SABRE professional. 

A full security design methodology is a comprehensive approach to security design involving a detailed risk assessment and design process that is recommended for complex projects where security is critical. This integrates qualified security consultants throughout the entire project life-cycle (planning, design, construction, handover), and tasks are often mapped to the RIBA Stages.

Standard design applies pre-determined security measures based on industry standards, but may not be suitably tailored to specific project risks. This is often adopted for simpler projects where security isn’t a primary concern. Typically employed by building services engineers, lacking secure risk assessment. This may risk incomplete security solutions and misaligned measures, as well as potentially unnecessary costs.

A defined planning process may be set by local authorities with specific security requirements and will vary by jurisdiction.

Reducing all risks to zero is often impractical. Therefore, risk management aims to reduce risks within the risk owner’s acceptable range, typically to ‘as low as reasonably practicable’ (ALARP). While security regimes cannot directly influence threat levels, they can significantly impact vulnerability and potential consequences. This is where the focus of risk management lies – reducing vulnerabilities through physical, technical, and operational measures (including user management). This relies on continuous communication, coordination, monitoring, and review. This iterative approach ensures risk mitigation measures are effective and residual risks are understood. 

Until such time as these leading practices become more widely adopted throughout the development industry, it is likely that building services engineers may still be requested to undertake security engineering activities. This practice, and the reliance on procured physical and technical security solutions in the absence of a risk-based strategy, should be discouraged.

If a building services engineer — or even a security engineer — is procured only to provide security systems design at RIBA Stages 3 or 4, it is likely that there will be significant gaps in the overall strategy, as the solutions will not be based on a sound foundation for technical systems design and will not be integrated with the physical and operational security measures. As a result, it will not be possible to confidently state that the project’s security is holistic, balanced, proportionate to the risks, appropriate to the context, and effective at reducing vulnerability and consequences. 

On its own, a building services engineering approach may not be able to demonstrate that security is fit for purpose, as there is nothing against which to benchmark design or measure the efficacy of the security solution.

This results in simply having security that may be good, but which is unable to be judged positively or defended in any meaningful way, or contribute to the ‘golden thread’ of assurance through the project by tracking risk, mitigation, and residual risk monitoring.

Further reading
CIBSE AM 4.1 Security engineering: Strategy

References:

1. BS ISO 31000:2018: Risk management. Guidelines, British Standards Institution 2018.
2. bit.ly/CJFeb24CPD1 – accessed 1 January 2024.
3. Secured By Design: bit.ly/CJFeb24CPD2 – accessed 1 January 2024.
4. bit.ly/CJFeb24CPD3 – accessed 1 January 2024.

The post Module 229: Security risk management processes for building services appeared first on CIBSE Journal.

The network of power, control, and communication cables that pass through every part of a building provide amazing utility, but if not properly specified and procured, they could equally create a significant hazard and risk to safety in the event of a fire. 

This CPD will consider how significant UK fire events have heightened concerns in building safety, discuss some of the principal UK standards that are defining fire performance in cables, and identify some of the potential gaps that might well be filled in the revitalised, responsible culture ushered in by the recently introduced Building Safety Act¹ in England.

As reported in CIBSE Journal June 2018, the inquiry in the aftermath of the Grenfell Tower fire that occurred in London on 14 June 2017 determined that all lobbies from level 4 to level 23 became smoke-logged and, repeatedly in the testimonies of the Grenfell survivors, black smoke (Figure 1) had impeded safe escape. The 2009 fire in Lakanal House, Southwark, was reported to the Grenfell enquiry, as there were thought to be similarities in the response to the fire and the rapid spread of the fire to other floors, as well as the devastating impact from smoke.

Figure 1: Grenfell Tower at 4.45am on 14 June 2017 (Source: Nathalie Oxford – bit.ly/CJJan24CPD5)

Within 30 minutes of the fire starting, smoke had spread to involve floors 6 to 12 (of the 14 storeys) and smoke-logging affected large parts of the building, including the communal staircase, corridors and many of the flats. However, some 22 years prior to that, the King’s Cross Underground station fire in London (Figure 2) had already heightened concerns around fire safety procedures and protocols, and the devastating impact of smoke on safe egress from a fire. In addition to loss of life, the fire destroyed much of the station equipment and fixtures, including the cabling systems.

However, the predominant cause of death resulted from the dense black smoke.² Witness statements in the subsequent public inquiry² noted that as survivors ‘reached the steps up to St Pancras Station, the smoke quickly turned from brown to dense black, which smelt to them like a burning plastic cable’. The coroner reported that it was impossible to ascertain the source of toxic fire fumes, and so it was not possible to determine the source of toxic materials found in the bodies of those who had died.²

In these tragic fires, electrical wiring was not directly implicated in the cause of the fire, or in specifically creating the smoke that prevented the safe escape of many occupants. At King’s Cross, for instance, a fire-damaged cable had disrupted the automatic operation of the Victoria line, but this was not explicitly associated with the main incident.

In all three fires, multiple factors coincided to produce the catastrophic outcomes. Many of these shortcomings were undoubtedly identified in the respective inquiries, and there has been significant work undertaken in the intervening years by London Underground/Transport for London, the London Fire Brigade, and, most recently, through the designation of responsibility and competence (and the aspiration of ‘cultural change’) in the Fire Safety Bill 2021³ and the Building Safety Act 2022.

A key objective of the acts is to remedy the systemic issues identified by the Independent Review of Building Regulations and Fire Safety⁴ by strengthening the whole regulatory system for building safety. Many of the new requirements apply not just to ‘high-risk buildings’ but to all non-domestic premises, such as where people work, visit or stay, including workplaces, and the non-domestic parts of multi-occupied residential buildings (for example, communal corridors, stairways and plantrooms). The requirements do not apply within individual domestic premises.

Figure 2: Firefi ghters emerge through the smoke as it billows from the fire at King’s Cross Underground station (Source: London Fire Brigade – bit.ly/CJJan24CPD4)

As seen in the fires discussed above, large, complex, and high-rise built environments that are more likely to be densely populated will have extended evacuation times. Smoke from inappropriately specified or inadequately manufactured cables will reduce the opportunities for successful escape from a fire. Circuits of safety-critical services need to function for extended periods, and fire plans are likely to rely on critical circuits continuing to perform to prevent potentially disastrous events, such as: fire alarm cable failure; sprinkler system not activating; smoke extract fans and smoke louvre power supply failure; and emergency lighting and signage failing to remain illuminated.

Fire-resistant cables provide extended periods of circuit integrity where uninterrupted functionality is crucial during a fire. They are designed to maintain their functionality and structural integrity for a specified period of time during a fire, and are constructed using materials that can withstand high temperatures when exposed to tested levels of flame, water and shock. They remain intact in harsh conditions, although they are not necessarily fireproof.

The insulation and sheathing are made from materials that do not propagate flames or produce excessive smoke, and so are able to provide protection against fire. Performance will vary depending on the specific type and design of the cable. The two principal types of fire-resistant cables are polymeric cables and mineral insulated copper cables (MICC).

Polymeric cables use synthetic materials such as polyethylene (PE), polyvinyl chloride (PVC), cross-linked polyethylene (XLPE), mica tapes and other polymers for insulation and sheathing, such as in the simplified sketch in Figure 3.

The flexibility of these cables makes them relatively easier to handle and install. Polymeric cables can be designed with various material layers to ensure that they maintain circuit integrity for a specified duration during a fire under defined conditions. The temperature rating of polymeric cables varies based on the specific polymers used.

Figure 3: A simplified sketch of an example fire-resistant polymeric cable. There are many variants of such cables that are designed to meet specific fire-resistance requirements

The Institution of Engineering and Technology (IET) notes⁵ that there are many acronyms employed to represent the emissions performance of polymeric cable, and it is important not to confuse ‘low smoke halogen free’ (LSHF) and ‘low smoke and fume’ (LSF). PVC compounds are used during the manufacture of LSF cables, and while additional additives reduce the smoke emissions, they are not eliminated. There are no standards governing LSF cables, unlike LSHF, which are manufactured and tested to BS EN 61034,⁶ which considers the measurement of smoke density from burning cables, and BS EN 60754,⁷ which provides guidance on corrosive and acid gas emissions.

In MICC, the conductors are surrounded by mineral insulation, commonly magnesium oxide (MgO), and the principal outer sheath is typically made of metal, such as copper (Cu) or an alloy, as illustrated in Figure 4 and Figure 5. They are rigid and less flexible compared with polymeric cables; however, the rigid construction, resulting from the highly compressed powdered mineral insulation, provides excellent mechanical strength.

MICC are inherently fire-resistant because of the mineral insulation, and can withstand high temperatures and maintain circuit integrity during intense fires. This type of cable can typically safely carry an electrical load at temperatures in excess of 1,000°C.

Figure 4: Examples of MICC (Source: Wrexham Mineral Cables)

Most cables installed as part of a permanent installation within domestic, residential and commercial buildings are subject to the Construction Products Regulation⁸ (CPR) that requires relevant cables to be CE-marked. Cable conformance includes reaction to fire and release of dangerous substances in normal operation, dismantling and recycling. All UK countries will accept⁹ the EU’s CE mark as appropriate for cables until at least 2025.

The supporting standard BS EN 50575¹⁰ covers the reaction to fire of cables in construction works on a scale of A_ca (non-combustible, such as bare MICC) to F_ca (no performance determined and likely to burn uncontrollably in a fire). In addition, there are classifications for smoke (s), flaming droplets (d), and acidity (a) as described in BS EN 13501-6,¹¹ with each classification graded from 0 or 1 to 3. The BCA provides guidance¹² on appropriate CPR ratings – for example, a cable designated as C_ca – s1, d2, a1 is likely to be suitable for installations where ‘improved’ (as opposed to ‘low-risk’) fire performance of cable is required.

CPR covers both reaction to fire and resistance to fire, but only the harmonised standard BS EN 50575, which considers reaction to fire, is currently available. Therefore, fire-resistant cables cannot be certified under the CPR, so it is not possible to CE-mark and issue a declaration of performance (DoP) for them.

In the various UK Building Regulations, to be classed as fire-resistant cable, a cable’s construction must meet the British Standard appropriate to the cable type and application. There are many cable fire-performance standards – code of practice BS 8519¹³ provides a useful table that lists the appropriate cable categories and standards for specific applications.

Figure 5: The components of a typical MICC

Each standard has a variant of time and flame temperature, with some incorporating physical shock and water spray, in order to test cables under simulated fire conditions – these can provide a bewildering array of ‘standardisation’. These can range from a 15- to 120-minute rated cable tested at 842°C – designated as PH 15 to PH 120 cable under BS 50200¹⁴ (which considers unprotected cables less than 20mm diameter for use in emergency circuits) – to a three-hour fire-rated cable tested at 950°C to BS 6387¹⁵ (designated ‘category C’).

Code of practice BS 5839-1,¹⁶ which considers fire detection and fire alarm systems for buildings, makes recommendations for two levels of fire resistance – ‘standard’ (PH 30 with water spray) and ‘enhanced’ (PH 120, also meeting BS 8434-2¹⁷) for unprotected cable. BS 8434-2 is a 120-minute test that includes direct flame, mechanical shock and a water spray, all conducted on the same cable sample in the same period at a temperature of 930°C. Cables with standard fire resistance are deemed suitable for the majority of  applications.

However, cables of enhanced fire resistance are recommended where prolonged circuit integrity is necessary, such as in un-sprinklered high-rise buildings with phased evacuation arrangements, and premises that are likely to be part-occupied for a prolonged duration during a fire that might damage cables serving parts of the fire alarm system in occupied areas. Monitoring of circuits and protection of cables against damage are complementary requirement precautions, rather than alternatives.

Similarly, code of practice BS 5266-1¹⁸ recommends (for unprotected cable) ‘standard’ (PH 60 plus 30-minute water spray) cables for normal use in emergency lighting systems and ‘enhanced’ (PH 120, also meeting BS 8434-2) cables for use in certain large and complex buildings where circuits are required to operate for longer periods to aid evacuation.

The well-established BS 6387¹⁵ Test method for resistance to fire of cables required to maintain circuit integrity under fire conditions is widely applied internationally. The method is broken down into three separate tests – referred to as C, W, and Z – that are undertaken on a sample of cable.

C considers fire at 950°C; W, at 650°C, additionally includes a water spray with 15 minutes fire and 15 minutes sprinkler; and Z is carried out with the cable sample ‘rigidly’ mounted for 15 minutes at 950°C with a metal bar hitting the metal mounting frame every 30 seconds. Successfully passing the C, W, and Z tests outlined in BS 6387¹⁵ suggests to specifiers and contractors that a cable is capable of surviving all three scenarios of fire, water and shock.

However, there are features of the three tests that have been highlighted by manufacturers as potentially not being sufficiently stringent. These include the fact that the three tests do not have to be carried out on the same cable sample; the volume flow of water in test W is a minimum of 0.3 litres of water per minute – which compares with a real-life scenario fire hose discharge in excess of 500 litres per minute; and in test Z there is no direct impact on the cable sample – in a fire, debris is likely to impinge directly on cables.

In his recent review,¹⁹ cable fire safety expert Richard Hosier emphasises the global prevalence of the BS cable flame test methods for certifying fire-resistant electrical cables. He highlights the standard 500mm-long gas ribbon burner test rig, expressing concerns that the set flame temperature in ‘open air’ tests may not consistently match the full cable temperature. Hosier compares UK standards with international practices, noting that furnace testing – exposing the full cable specimen to furnace temperatures – is adopted in other ‘developed’ countries, aligning with fire-resistance testing requirements for various building elements.

The Building Safety Act is establishing a fresh regulatory framework, placing accountability on those involved in procuring, designing, creating and maintaining buildings to ensure safety for occupants. Moreover, it grants consumers the authority to pursue legal action against manufacturers and suppliers for breaching the CPR or providing misleading information during product marketing or supply.

Building Regulations requirements should be considered as a minimum standard. The selection of an appropriately tested and independently certified cable requires that the manufacturer, supplier, designer, and installer have a robust understanding not only of the application, but also the practical fire performance that might be expected from the cable based on the necessary limitations of the standardised testing procedures.

References:

1. Building Safety Act 2022.
2. Fennel, D, Investigation into the King’s Cross Underground Fire, UK Department of Transport, 21 October 1988.
3. Fire Safety Bill 2021.
4. Independent Review of Building Regulations and Fire Safety.
5. bit.ly/CJJan24CPD1 – accessed 26 November 2023.
6. BS EN 61034-2:2005+A2:2020 Measurement of smoke density of cables burning under defined conditions – Test procedure and requirements, BSI 2020.
7. BS EN 60754-1:2014+A1:2020 Test on gases evolved during combustion of materials from cables – Determination of the halogen acid gas content, BSI 2020.
8. Regulation (EU) No 305/2011 of the European Parliament and of the Council of
   9 March 2011 laying down harmonised conditions for the marketing of construction products and repealing Council Directive 89/106/EEC.
9. bit.ly/CJJan24CPD2 – accessed 26 November 2023.
10. BS EN 50575:2014+A1:2016 Power, control and communication cables. Cables for general applications in construction works subject to reaction to fire requirements, BSI 2016.
11. BS EN 13501‑6:2018+A1:2022 Fire classification of construction products and building elements Part 6: Classification using data from reaction to fire tests on power, control and communication cables, BSI 2022.
12. Recommendations for the Selection of Cables under the Construction Products Regulation (CPR), British Cables Association 2019.
13. BS 8519:2010 Code of practice for the selection and installation of fire-resistant power and control cable systems for life safety and fire-fighting applications, BSI 2010.
14. BS EN 50200:2015 Method of test for resistance to fire of unprotected small cables for use in emergency circuits, BSI 2015.
15. BS 6387:2013 Test method for resistance to fire of cables required to maintain circuit integrity under fire conditions, BSI 2013.
16. BS 5839-1:2017 Fire detection and fire alarm systems for buildings Part 1: Code of practice for design, installation, commissioning and maintenance of systems in non-domestic premises, BSI 2018.
17. BS 8434-2:2003+A2:2009 Methods of test for assessment of the fire integrity of electric cables – Test for unprotected small cables for use in emergency circuits. BS EN 50200 with a 930° flame and with water spray, BSI 2009.
18. BS 5266-1:2016 Emergency lighting – Code of practice for the emergency lighting of premises, BSI 2016.
19. bit.ly/CJJan24CPD3. 

The post Module 228: Contributing to fire safety in buildings with suitably specified cables appeared first on CIBSE Journal.

As a new tenant takes over a section of a commercial building, whether a new construction or an existing tenanted premises, it is very likely that their specific requirements will require a reconfiguration and refitting of the space to suit their particular needs. This process, known as a ‘fit-out’, can result in significant expense both financially and environmentally, as hardware such as partitions, furnishings, fixings, lighting, IT, and environmental systems that were previously useful assets to the previous tenant are variously altered, removed or replaced.

One common element that contributes to this potentially profligate process is the fan coil unit (FCU). This CPD will consider the fit-out process, and assess some options that may improve the environmental and financial impacts of fitting out commercial buildings to satisfy tenant needs.

According to the Royal Institution of Chartered Surveyors (RICS),¹ around 11% of total construction expenditure in the UK is allocated to fit-outs. RICS suggests that buildings may undergo as many as 30 to 40 fit-outs over their life-cycle. For a new building shell, or one that has been completely refurbished, the Cat A fit-out will typically include essential elements necessary for occupancy but still provide a ‘blank canvas’ ready for the tenant to individualise it.

It would normally encompass elements including basic (infrastructure) electrical, plumbing and mechanical services, raised access flooring, finished wall coverings and suspended ceilings, and encompass the provision of hallways, staircases, lifts, and toilet facilities. Air conditioning may be included during this phase. In the commercial sector, there are several heating, ventilation, and air conditioning (HVAC) solutions available, but FCUs are a common choice because of their ability to offer tenants zoned control over the indoor temperature.

During this stage, open-plan areas are typically extensive and require the selection of large and powerful FCUs to meet the demand for conditioned air. FCUs serving areas near glazing around the building’s perimeter are sized to handle both heating and cooling requirements. In central areas where heat loss is minimal, FCUs are often specified for cooling purposes only.

After the property space has been leased, the Cat B fit-out customises the area to suit the specific needs of the tenants. This involves reconfiguring the interior layout to create working spaces, reception areas, kitchen facilities and meeting rooms. It also includes the installation of all the necessary IT, audio-visual equipment and lighting systems.

These alterations can have an impact on the heating and cooling requirements, and it is common for the original placement of the FCUs from the Cat A fit-out to no longer be optimal for serving the requirements of the new layout. The larger FCUs originally installed may now be inappropriate to condition the air in what is likely to be collection of smaller spaces. Most FCUs are typically selected to operate at about half fan speed to allow some variation above and below to cope with fluctuations in demand.

However, FCUs do not operate most effectively at low speeds, so with prolonged operation at very low speeds (to meet the smaller load) they will not operate with optimum performance.

As a consequence, some of the larger FCUs may be removed during a Cat B fit-out and possibly replaced by smaller FCUs that are better suited to cater to the requirements of the updated layout. These larger FCUs could potentially find use elsewhere in the project but, in many cases, they are no longer needed. The manufacturers typically do not accept these units for return, since they have been previously installed, commissioned, and have already been in operation during the Cat A phase – essentially rendering them second-hand.

Contractors might choose to retain the large FCUs and store them for potential use on other projects; however, it is likely that a significant proportion end up as scrap for metal recycling. The process of removing the original FCUs not only consumes time and money but also generates additional embodied carbon and waste. At the conclusion of the lease period, new tenants may require modifications to the layout, necessitating the repositioning of existing FCUs or the acquisition of new ones to effectively manage the heating or cooling demands of the space.

In certain cases, particularly when disputes between landlords and tenants lead to lease terminations, landlords might insist on restoring the building to its original condition. This typically involves the removal of the FCUs and the installation of new, larger FCUs according to the building owner’s original specifications. However, these larger FCUs might be considered as somewhat temporary and may be replaced when the space is leased again. This cycle further exacerbates carbon emissions and contributes to construction waste.

There are several options that could help prevent this wasteful – and likely repeated – cycle of FCU removal and replacement, as discussed below.

The first option is that the project could bypass the Cat A stage and proceed directly to the Cat B phase. However, this approach demands a high level of collaboration during the design phase, and is only feasible when the ultimate client for the building has been identified. This scenario becomes possible when, for instance, a large corporation makes the decision to both construct and occupy a building or commits to a long-term lease of the property.

While this does occur, it remains uncommon and not without its challenges. The Cat B specification must be fixed relatively early in the project and, consequently, any design modifications necessitated by shifts in the company’s strategy, structure or size can introduce their own set of issues if not carefully managed.

Alternatively, an option is to utilise a greater number of smaller FCUs, offering enhanced inherent flexibility and potentially decreasing the waste generated during removal and replacement that might otherwise be needed. Nevertheless, while this approach could diminish the likelihood of having to revisit the FCU strategy during the Cat B phase, it would lead to increased costs for the building owner while reducing the tenant’s Cat B expenses.

Each smaller FCU would also involve the installation of a controller, valve set, pipework and electrical connections. Increased numbers of FCUs will also add to the complexity of structural coordination when setting out a reduced size zone (or ‘bay’). While the use of smaller FCUs can enhance the property’s appeal and make it more attractive to prospective tenants, the initial cost linked to procuring and installing additional FCUs may pose challenges when justifying this expenditure to the building owner.

A more radical option, for larger multi-floor projects, is to fit-out only some floors as Cat A and fit-out the remainder of the floors to Cat B once tenants are secured. However, a direct Cat B fit out may then require the complete infrastructure pipework, ductwork and wiring to be installed so the tenant may have a longer wait to make proper use of their space. This may deter some clients for whom time is of the essence. However, it is a likely to provide a more sustainable, and potentially cost-effective, approach.

The final option, to be discussed in this article, provides a novel FCU system that can be installed to fully meet the needs of Cat A and then be adapted to allow reuse (with, mainly, ductwork and diffuser alterations) to completely satisfy tenant requirements at Cat B. A ‘multizone’ FCU – which is described more fully below and pictured in Figure 1 – can supply up to five zones, each of which may be operating with different temperature setpoints and have different load profiles. Although similar in external appearance, this is quite different to a traditional single-fan or multi-fan FCU with multiple spigots, as that will offer no opportunity to continuously control the proportion of the total flowrate to the separate outlets. 

Multizone FCU

Advances in the design of compact digitally controlled electronically commutated (EC) motor-driven fans enabled the development of a single FCU housing that incorporates multiple, partitioned, independently controlled direct-drive EC fans, which supply separate zones, drawing from a common plenum of treated (cooled/heated) air (as illustrated in Figure 2). 

The units typically contain a multi-row cooling coil and a heating coil. The speed of the fans may be individually controlled by the integrated controller that interrogates all the separate zone temperature sensors and assigns a priority zone. This will be the zone that is furthest from its set-point and, for example, could require cooling. With the multizone FCU unit in cooling mode, the priority space receives the full design air volume flowrate, with other cooling zones receiving proportionately the air flowrate they need to satisfy their lower cooling requirements. Any space that requires heating at that time has its fan stopped and no air is delivered. Once the cooling demands have been satisfied, and if there is still a need for heating, then the cooling coil valve closes and after a purge period the cooling zones fans stop. The heating coil valve is opened and the heating zone(s) fans activate. Once the heating requirement is settled, the unit will revert to cooling mode and the cycle may then continue. The multizone unit would typically be located to serve similar thermal zones. The FCU integrated controller optimises the cooling and heating water flowrates and the individual fan speeds to deliver the required zone temperature control in the most energy-efficient way.

In the unit illustrated in Figure 1, the heating and cooling control valves are electronic pressure independent valves (EPIV). The EPIV provides the same function as a pressure independent control valve (PICV), as discussed in CIBSE Journal CPD Module 140; however, the EPIV achieves this with a close-coupled temperature compensated, inline ultrasonic flowrate meter that provides a signal of the water flowrate to the valve actuator controller. The required flowrate is determined and sent to the actuator by the FCU integrated controller. This allows the valve to continuously modulate to provide the correct water flowrate. An EPIV will operate at lower pressures than that required by a PICV – typically from 1 to 15kPa, depending on the system peak load (compared with 20-30kPa for a typical PICV). The signals from the EPIVs and the fan speed controllers can, through the integrated FCU controller, provide information to the building energy management systems (through protocols such as BACNet) for monitoring, recording, optimisation and preventative fault diagnosis.

The single-unit multizone FCU requires just one set of pipework and wiring, compared with the network of connections to multiple traditional FCUs, and significantly reduces the cost and environmental impact of installing and otherwise potentially replacing traditional FCUs.

Multizone FCUs reduce technology waste not just for the first Cat A to Cat B transition but also for all the ones that will follow, as tenants leave, and can provide significant saving in cost, time, and waste. This negates the need for replacement FCUs that will otherwise add more embodied carbon and increase the building’s whole-life carbon footprint.

A single multizone unit can be installed as part of Cat A fit-out and initially used so that all the fans and outlets work in unison to serve the large undivided space – known as multizone-ready. This would be positioned so that it is likely to eventually serve zones with similar thermal load profiles. During Cat B, the ductwork can be reconfigured to serve diffusers in each of the newly partitioned spaces.

The multizone FCU may be located outside of the controlled zones allowing, for example, higher ceilings in the remaining areas, with fewer access panels and less space needed. The control arrangements for the rooms serviced by a multizone FCU are flexible, ranging from individual controls in each zone through to being centrally set through the building management system (BMS).

The traditional cycle of fit-outs is inherently wasteful. Establishing the demands of at least a proportion of the initial building tenants can provide opportunities for a reduction in the amount of Cat A fit-out that is then subsequently replaced and potentially wasted. If the building environmental systems are designed to be readily adaptable to accommodate a wide range of building uses, this will not only reduce the scale of necessary modifications but also potentially ease the transition between tenants.

Assessing the application of adaptable multizone FCUs at the early stages of the systems design could endow future building tenants with a cost effective, efficient, and flexible cooling and heating solution that can also significantly reduce the lifetime embodied carbon compared with maintaining a fit-out cycle of removing and replacing traditional FCUs.

• This article has drawn extensively from the white paper Reducing waste in development fit outs through effective fan coil solution design, produced by Ability and independent consultants.

References:

1. bit.ly/CJDec23CPD21 – accessed 6 November 2023.

The post Module 227: Reducing fit-out waste and the carbon footprint of fan coil unit installations appeared first on CIBSE Journal.

It is seven years since CIBSE Journal produced a CPD article specifically on the application of refrigerant R290 – propane – that, at the time, was likely seen by many as an outlier in the refrigerant marketplace. Over the intervening years, the relative benefits, challenges and opportunities of using such refrigerants – one of the ‘natural’ refrigerants – have moved on significantly. This CPD will highlight the continuing challenges, and consider the changes that appear to be ushering in the new era of propane-charged heat pumps and chillers.

The quest for more environmentally benign refrigerants gained international prominence when it was discovered that the synthetic refrigerants chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) were adversely impacting the global ozone layer. Under the 1987 Montreal Protocol,¹ many countries agreed to phase out CFCs and HCFCs. This accelerated the application of hydrofluorocarbons (HFCs) that – although exhibiting zero ozone depletion potential (ODP) as they have no chlorine content – were subsequently associated with global warming.

The 2016 Kigali Amendment set a schedule for countries to gradually reduce HFC consumption, initially with developed nations taking the lead, and now recognised by 155 countries.² The global initiatives to reduce the use of high-global warming potential (GWP) refrigerants have a significant focus on the reduction in the use of fluorinated HFCs and hydrofluoroolefins (HFOs). In Europe the F-gas regulation,³ initiated in 2016, set the pace, and this has provided the basis of the subsequent UK F-gas regulation. 

Recently, in October 2023, revisions to the EU F-gas regulation were provisionally agreed⁴ to accelerate the implementation of measures towards the phasing out of HFC consumption by 2050, with the production of HFCs phased down to a minimum (15%) from 2036. A full ban was provisionally agreed, commencing in 2027, for specific chillers, small (<=12kW) monobloc heat pumps, and air conditioning with F-gases with a GWP >= 150, with complete F-gas phase out in 2032.

For split air conditioning and heat pumps that contain F-gases, the agreement was for a full ban starting in 2035, with earlier deadlines for systems with higher-GWP refrigerants. There was some wriggle-room included to support the aspirations of the EU to significantly increase the adoption of heat pumps – most of which currently use HFCs. 

However, the suggested revisions will have a very significant impact on the selection of refrigerants and systems. Notably, the revisions also introduce certification schemes covering the safe handling of natural refrigerants. Simultaneously, the US Environmental Protection Agency⁵ announced enhanced provisions for the reduction and reuse of HFCs that will apply to products both produced in the US and imported. This will undoubtedly influence the global marketplace.

Separately, a recently completed consultation⁶ by the European Chemicals Agency considered proposals to restrict the use of per- and polyfluoroalkyl substances (PFAS) across the EU. The inclusion in the potential banned list of single component gases R125, R134a, R143a and the HFOs R1234yf and R1234ze(E) affects virtually all new and current lower-GWP HFC/HFO refrigerant blends.⁷ This has excited a huge response, including from trade organisations representing the refrigeration and air conditioning sector, who claim that the timeline of proposed reductions is practically impossible to meet.

As reported⁸ recently by the UK Committee on Climate Change (CCC), following the UK’s commitment to the 2008 Climate Change Act⁹ and the 2015 Paris Agreement,¹⁰ the UK government aims to reduce F-gas emissions to less than 3.4MtCO_2e by 2035, from 11Mt in 2021, with most of the planned reduction coming from the UK F-gas regulation.

UK F-gas emissions have fallen over the past few years, decreasing by 6% in 2021; however, emissions remain higher today than in the early 2000s, and only 26% lower than 1990 levels. The CCC has determined that the consumption of HFCs must decrease to 15% of 2015 levels by 2035 to meet the UK government’s target. The UK F-gas regulation provides the mechanism to reduce this if, as CCC notes, it is successfully enforced.

The CCC also recognises the risk that emissions may increase with the roll-out of heat pumps, which currently mostly use F-gas refrigerants, unless the UK government takes action to ensure that there is a shift to non-F-gas refrigerants (such as propane, R290 and CO₂, R744). Although the UK government has committed to reviewing the UK implementation of the F-gas regulation, there is, as yet, no clear legislative timeline, and no indication as to whether it will follow the lead in the recent provisionally agreed changes to the EU F-gas regulation

However, industry has not stood still. There has been a significant transition away from the use of R-410A (that was originally developed to displace the high-ODP refrigerants, such as the lower-pressure HCFC R22, chlorodifluoromethane) to HFC R32 and the HFO/HFC blends, such as R454B.

The synthetic HFOs typically have a 100-year GWP of between just 1 and 4. R32 (difluoromethane) had a previously accepted GWP of 675, but this has recently risen to 771, while R454B has a GWP of 467. One of the key goals is to produce refrigerants that have favourable thermodynamic properties, including relatively low saturated vapour pressures. Many very closely emulate the historically favoured refrigerant HCFC R22, as illustrated in Figure 1.

Although these are all energy-efficient refrigerants, most have other less welcome attributes that are likely to limit their eventual application in building services. The recent report¹³ from UK government confirmed that there has already been a significant transition away from the use of R410A to R32 and HFO/HFC blends, noting that R32 has proved to be an important alternative, with similar characteristics to R-410A, apart from it being a ‘lower flammability’ refrigerant – designated as A2L under BSI ISO 817.¹⁴

(R410A is designated as an A1, non-flammable refrigerant.) The F-gas regulation contains an upcoming ban on small split air-conditioning systems with less than 3kg charge using F-gases with a GWP of 750 or more, from 1 January 2025.

Although the UK report indicates there is already some use of propane, it is very limited. However, there is a significant upturn in major manufacturers’ interest in ‘natural’ refrigerants – which includes propane, as well as CO₂ and potentially ammonia (R717) – for use in new air conditioners and heat pumps. This is motivated by the deleterious environmental impact of many synthetic refrigerants and the lower GWPs of propane and CO₂ and, pragmatically, has been accelerated by regulatory requirements. 

As discussed more fully in the CIBSE Journal CPD Module 99 from December 2016, propane (as well as other HCs) typically exhibits low pressure drops and achieves system efficiencies that are equal to – or exceed – those of synthetic alternatives. The latent heat of vaporisation of propane is twice that of the most common HFC refrigerants, so providing a higher cooling/heating effect for the same refrigerant mass flow.¹⁵ 

Work by the Fraunhofer Institute and a group of manufacturers¹⁶ has determined that heat pumps are practically operable with a refrigerant charge of about 10g of propane per kW, as compared with about 100g of propane per kW required with typical designs. Systems are already evolving to reduce refrigerant use – the example of the monobloc heat pump in Figure 2 has a 14.3kg charge to provide up to 195kW of heating and about a quarter of the refrigerant mass required for comparable R410A units.

Figure 2: Commercial, monobloc heat pump with a 14.3kg charge of R290 (propane), to provide up to 195kW of heating (Source: Swegon)

The thermodynamic qualities of propane enable operation at low evaporating temperatures and high condensing temperatures, allowing it to provide water temperatures beyond 65°C (at sub-zero external temperatures) with COPs that could go as high as 4.5 (but are practically somewhat lower than that at the higher temperatures).

Propane is a colourless, odourless gas with a very low GWP (see boxout ‘The changing GWP’), an ODP of 0, and low toxicity (designated ‘A’ under BS ISO 817). Most propane is produced from liquid components recovered during natural gas processing and during crude oil refining (alongside other chemicals including ethane, methane and butane).

Although the amounts are relatively small, ‘renewable’ propane is also produced from biomass-based feedstocks, including used cooking oil, animal fats, or dimethyl ether (which is also one of the original 19^th-century refrigerants). At atmospheric conditions, propane is a gas that is denser than air.

Propane is easy to procure and relatively cheap in price; however, the greatest weakness of propane, as with all HCs, is that it is highly flammable and so designated a ‘3’ under BS ISO 817. Flammable hydrocarbons require careful consideration of safety when applied in systems, and any installation should meet the requirements of standard BS EN 378-3.¹⁷

Units are constructed to stringent guidelines, employing ATEX (‘Atmosphere Explosive’ EU Directive 2014/34)-rated components and segregated electrical compartments to prevent spark risks. Dedicated leak-detection systems are used to monitor levels of propane that provide purging to outdoors, employing ATEX-rated extract fans, to ensure that the levels do not rise to approach the lower flammable limit (LFL) of propane (2.1% by volume of air – compared with 1.4% for petrol and 5% for natural gas). Being denser than air, any leaked gas has the potential to pool at low level and so systems should not be sited near drains or pits. Although not toxic, propane has caused deaths through asphyxiation.

Following an extensive review process, IEC 60335-2-40¹⁸ – which deals with the safety of heat pumps – was revised in 2022 to increase the refrigerant charge limit in standard split air conditioning applications for use inside the buildings (it is currently under BSI review). The limit for R290 was increased from 340g to 988g in new equipment so long as it incorporates additional safety requirements to provide the same level of safety as equipment using non-flammable refrigerants. The new limits could allow more than 13kW of heating from split units.

The UK government’s assessment of the HFC phasedown²¹ suggests a struggle to meet the required reductions and timeframes. The marketplace for air conditioning and heat pumps is predicted to change significantly over the next 25 years, as shown in Figure 3. This emphasises the importance of a swift roll-out of efficient, low-GWP refrigerants for residential and commercial applications.

The regulatory authorities are enthusiastically promoting the practical use of R290 with increases in allowable refrigerant charges, while researchers and manufacturers are creating systems with increasing efficiencies and reducing refrigerant charges. With installations designed, installed and operated to meet stringent safety requirements, prospects appear to be good for a massive expansion in the number of propane heat pumps and air conditioners that can provide increasingly effective systems with low operational environmental impact.

References:

1. bit.ly/CJDec23CPD1 – accessed 1 November 2023.
2. bit.ly/CJDec23CPD2 – accessed 1 November 2023.
3. Regulation (EC) No 842/2006 of the European Parliament and of the Council of 17 May 2006 on certain fluorinated greenhouse gases.
4. bit.ly/CJDec23CPD8 – accessed 1 November 2023.
5. bit.ly/CJDec23CPD3 – accessed 1 November 2023.
6. bit.ly/CJDec23CPD4 – accessed 1 November 2023.
7. bit.ly/CJDec23CPD5 – accessed 1 November 2023.
8. Progress in reducing emissions – 2023 Report to Parliament, UK Climate Change Committee, June 2023.
9. bit.ly/CJDec23CPD6 – accessed 1 November 2023.
10. 21st Conference of the Parties (COP 21), Paris, 2015.
11. Intergovernmental Panel on Climate Change (IPCC), Climate Change 2021 – The Physical Science Basis Working Group I contribution to the Sixth Assessment Report
12. bit.ly/CJDec23CPD7 – accessed 21 October 2023.
13. F-gas regulation in Great Britain Assessment report, DEFRA, December 2022.
14. BS ISO 817:2014+A2:2021 Refrigerants. Designation and safety classification, BSI 2021.
15. bit.ly/CJFeb23R2901 – accessed 1 November 2023.
16. Propane-based refrigeration circuit for heat pumps achieves new efficiency record, Fraunhofer ISE, 2022, bit.ly/CJFeb23R2902 – accessed 1 November 2023.
17. BS EN 378-3:2016+A1:2020 Refrigerating systems and heat pumps. Safety and environmental requirements – Installation site and personal protection, BSI 2020.
18. IEC 60335-2-40:2022 Household and similar electrical appliances – Safety. Part 2-40: Particular requirements for electrical heat pumps, air-conditioners and dehumidifiers, IEC 2022.
19. BS EN 378-2:2016 Refrigerating systems and heat pumps. Safety and environmental requirements – Design, construction, testing, marking and documentation, BSI 2016 (under review).
20. Eckhoff, R, et al, On the minimum ignition energy (MIE) for propane/air, Journal of Hazardous Materials, March 2010.
21. F gas regulation in Great Britain – Assessment report, UK DEFRA, December 2022.

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In a recent UK government research briefing,¹ heat pumps were reconfirmed as a key technology for achieving net zero emissions from domestic heating. However, the UK government target of 600,000 installations per year by 2028 still appears somewhat ambitious, being that just 72,000 were installed in 2022.

The research briefing asserts that heat pumps are technically suitable for most UK homes if installed appropriately. However, although the supply chain has significant growth potential, stakeholders indicate it is currently constrained by a lack of consumer demand and government policy uncertainty. Some of this uncertainty and lack of demand is doubtless the result of heat pump trials undertaken earlier this century that, in many cases, suffered from poorly or inappropriately applied installations and lack of understanding, variously by designer, installer and operator.

This reluctance to apply heat pumps is not apparent in many European states – many of which have climates that are similar, or more extreme, than those of the UK. An interesting set of data was produced by the European Heat Pump Association (EHPA),² based on sales intelligence and methodology from EU Decision 2013/114/EU.³ This explored how many heat pumps were technically feasible across other European countries if heat pumps were as popular as they are in Finland.

These are reflected in the chart in Figure 1 – the green columns indicate the potential and the blue columns indicate the actual amount of renewable energy produced by the current stock of heat pumps. This data reflects all heat pump sales, and although the comparisons may not be absolute in their methodology, the strong indication is that there is much untapped potential still to be exploited in many countries.

The UK has⁴ around 412 heat pumps per 100,000 people, compared with a European average of 3,068 heat pumps per 100,000 people. The Climate Change Committee projects⁵ that to reach net zero, domestic heat pumps will be needed in at least half – but likely closer to 80% – of UK homes by 2050. There are approximately⁶ 27 million homes in the UK and 68 million people.⁷

Air source heat pumps (ASHPs) employ an air-to-refrigerant evaporator coil to directly extract heat from a fan-assisted airflow. This coil features fins on the airside and is typically situated outdoors, utilising the surrounding ambient air as a heat source, or potentially in an air stream that is warmer than outdoor, ambient temperature, such as an exhaust air duct.

The condenser is often a compact plate heat exchanger that draws heat from the condensing refrigerant to increase the temperature of water that is being employed to supply the heating load. (Such an ASHP would be known as an air-to-water or air-to-brine heat pump.) The coefficient of performance (COP) provides an instantaneous measure of refrigeration cycle performance.

To achieve the best COP, the source (air passing over evaporator) should have as high a temperature as possible and the condenser temperature – governed by the temperature of water returning from the heating load – as low as practicably useful. To allow comparisons between systems and regulatory limits, the conditions for the measurement of the COP are typically standardised – for example, Approved Document Part L of the England Building Regulations (AD L England) refers to the methods of BS EN 14511-2.⁸

The seasonal COP (SCOP) is an aggregated COP over a year and, again, the calculation employs prescribed sets of external climate (temperature) bins and operating hours – these are defined in BS EN 14825:2018.⁹ SCOP values typically range from 3.0 to 5.0 for modern ASHPs.

The seasonal performance factor (SPF) is often employed to assess the performance of the whole defined heat pump system aggregated and averaged across yearly operation. This accounts not only for the changes in COP as the evaporator and condenser temperatures vary, but also other de-rating factors – such as the de-icing cycles required to keep the evaporator clear of ice at low outdoor temperatures, and parasitic power used in fans and control systems.

Therefore, the SPF value includes all the relevant ancillaries associated with the particular building, whereas COP is associated with the heat pump unit itself. There are various defined sets of assumptions for ‘boundaries’ (of the equipment/environments to be included) in SPF assessments that will impact the calculated value. These are illustrated in Figure 2.

For air-to-water heat pumps, ADL England requires a COP of at least 3.0 for space heating, and 2.0 for domestic hot water (with prescribed sets of standard inlet and outlet conditions both for internally and externally located heat pumps). Heat pumps designated as low-temperature would not typically deliver heating water higher than 52°C, and medium- and high-temperature heat pumps are characteristically capable of typically delivering water at higher temperatures.

Where a heat pump is providing useful cooling as well as delivering heating, the overall performance will be significantly higher. 

The UK government-funded ‘Energy Systems Catapult’ electrification of heat demonstration project¹⁰ installed 742 heat pumps in existing homes across Great Britain in 2020/21. The recently published interim findings found that ASHP SPFs have significantly improved, by ~0.3 to 0.4 compared with earlier trial installations reported¹¹ in 2017, with the median SPF_H4 for ASHPs being 2.80 (median SPF_H2 = 2.94).

It is noted that heat pumps using the ‘low-GWP’ refrigerants R290 (propane) and R32 generally performed better than those using the older R410a refrigerant (which is currently being phased out in the UK). This result may also suggest that the design (and installation) of heat pump systems has also improved. However, it also found a need to improve the quality and consistency of heat pump designs and installations to support a large-scale rollout of heat pumps in existing homes, and deliver positive energy, carbon and consumer outcomes.

Median ASHP efficiency fell to 2.44 on the coldest day (-0.4°C), which quantifies the expected degradation in performance resulting from low temperature, and could be used to inform modelling of peak winter demand. Notably, the best-performing installations had the largest proportion of the annual load met solely by heat pumps (with lower use of supplementary systems).

Exhaust air heat pumps (EAHPs) are a specific application of ASHPs, combining an ASHP with a mechanical ventilation system. As noted in BSRIA BG 7/2009,¹² exhaust air is an alternative heat source to outdoor air for buildings designed with mechanical ventilation.

The exhaust air is always at the indoor air condition, and so the source-to-load temperature difference is relatively small. The evaporator also gains from the latent heat in the exhaust air. The exhaust air may be drawn through the dwelling via transfer grilles or ducted from different areas including wet areas (but not from cooker hoods). 

EAHPs are a form of mechanical heat recovery, but are many times more effective than simple air-to-air heat exchangers or runaround coils. EAHPs may be employed to provide heating, domestic hot water (DHW) and mechanical ventilation in domestic and small commercial applications, via a single integrated unit that requires no external plant or equipment. They are best suited to buildings with high thermal performance and low leakage envelopes. If required, complementary heating can be provided from either a supplementary heating system or through resistive-electric heating.

The associated mechanical ventilation system will likely be one of two types – centralised mechanical extract ventilation or balanced supply and extract ventilation. For centralised mechanical extract ventilation-based systems, background ventilation is required through purpose-designed background ventilation devices that are intended to perform with EAHPs.

There is a common misconception that EAHPs – when operating as ‘four-pipe’ balanced ventilation devices, as illustrated in Figure 3 – follow the same principles as mechanical ventilation with heat recovery (MVHR) systems. However, unlike MVHR, where heat recovery and energy transfer is achieved by employing an air-to-air heat exchanger, the most common EAHP systems use an air-to-water heat exchanger to deliver heat from the heat pump condenser.  

In a ‘four-pipe’ balanced ventilation system, the incoming outdoor air is tempered using another water-to-air heat exchanger that is fed from the heating system. The efficiency afforded by the heat recovery is therefore incorporated into the overall SCOP for the heat pump, and not as a thermal efficiency rating that is commonly assessed for an MVHR system. 

The amount of available heat will be proportional to the extract airflow rate. To avoid over-ventilating the property – and, in turn, increasing air changes leading to higher heating loads – the ventilation system should be designed to meet the ventilation rates required to meet compliance with the local regulations (for example, in England, Approved Document Part F1, 2021).

To achieve higher EAHP heating outputs, it is possible to incorporate an additional, dampered, outdoor air duct that can be used to selectively draw outdoor air across the heat pump evaporator. Such hybrid source systems are recognised by the UK standard assessment procedure (SAP) PCDB (product characteristics database) as ‘mixed exhaust air and outside air heat pumps’. 

EAHPs require a hydronic heating emitter system, and most commonly incorporate an indirect unvented hot water cylinder. They work most efficiently at lower heating flow temperatures – as with other heat pump technologies – with distribution systems designed for delivering the required heating load at a maximum supply temperature of 55°C; this coincides with the most recent requirements in ADL England.

Most EAHP systems incorporate weather compensation control for the heating system and, potentially, inverter-driven compressors to modulate outputs. By employing refrigerants such as R290, EAHPs (such as in Figure 4) can deliver temperatures up to 70°C for the provision of hot water, allowing for legionella cycles to be carried out without relying on top-up electric heaters.

The heating delivered through either a hydronic heating system – such as underfloor heating or radiators – or, where balanced supply and extract mechanical ventilation is employed, potentially through both tempering the incoming air and supplying a hydronic heating system (as shown in Figure 3).

The specification and design of an EAHP system is critical to ensure that it operates efficiently and can provide 100% of the required heating and hot water demand. There have been several historic examples of trialled systems that were inappropriately matched to homes, which resulted in installations that were incapable of meeting loads without undue use of ‘backup’ direct electric heating.

A full heat-loss assessment should be undertaken and the DHW demand should be carefully assessed prior to considering any heat pump application. Based on current Building Regulations fabric standards, and depending on appropriate configuration, commercially-available packaged hybrid source EAHPs – with outputs such as those illustrated in Figure 5 – may be applied to meet the heating demands for new dwellings up to approximately 150m²; in the UK this is a typical three (or more)-bedroom house.

As noted in in the UK parliament guidance,¹ heat pump installation costs are higher than gas boilers, and large cost reductions are unlikely; however, heat pumps currently have similar running costs to gas boilers. With more creative tariffing, additional government initiatives, or greater energy security the relative gas and electricity cost differential may eventually erode – but, regardless, there is a prerogative to install heat pumps. For domestic and small commercial applications, properly integrated packaged EAHPs can provide one of the options to meet the ambitious target of 600,000 installations per year by 2028.

References:

1. bit.ly/CJNov23CPD1 – accessed 4 October 2023.
2. bit.ly/CJNov23CPD2 – accessed 4 October 2023.
3. EU decision on guidelines on calculating renewable energy from heat pumps from different heat pump technologies (2013/114/EU).
4. bit.ly/CJNov23CPD3 – accessed 4 October 2023.
5. The Sixth Carbon Budget The UK’s path to Net Zero, UK Committee on Climate Change December 2020.
6. bit.ly/CJNov23CPD4 – accessed 4 October 2023.
7. bit.ly/CJNov23CPD5 – accessed 4 October 2023.
8. BS EN 14511-2.
9. ABS EN 14825:2018.
10. Electrification of Heat Demonstration Project – Interim Insights from Heat Pump Performance Data – Energy Systems Catapult/Department for Energy Security and Net Zero (DESNZ) 2023.
11. Lowe, R et al, Final report on analysis of heat pump data from the renewable heat premium payment scheme, BEIS/ RAPID-HPC/UCL EI, 2017.
12. BSRIA BG 7/2009 Heat pumps – a guidance document, BSRIA 2009.
13. bit.ly/CJNov23CPD6 – accessed 4 October 2023.

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