CPD PROGRAMME | HEAT PUMPS Figure 2: SEPEMO (SEasonal PErformance factor and Monitoring) system boundaries when used to define the SPF boundaries, these are typically referred to as SPFH1 to SPFH5. Boundary H1 is the heat pump itself 11 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 INCLUDING HYBRID SOURCE EAHP IN SAP Within SAP modelling, an EAHP can be entered under multiple categories in order for the model to recognise all the different aspects of the system. This includes a listing as a ventilation device, and also a listing as a heat pump for main heating and hot water production. EAHPs are a recognised technology under the UKs Microgeneration Certification Scheme13 (MCS). 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 52C, 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 SPFH4 for ASHPs being 2.80 (median SPFH2 = 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 (-0.4C), which quantifies the expected degradation in performance resulting from low temperature, and could be used to inform modelling of peak winter demand. Notably, the bestperforming 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). Figure 3: Schematic of four-pipe exhaust air heat pump installation applied to a house with balanced supply and extract mechanical ventilation system (Source: NIBE) 22 November 2023 www.cibsejournal.com CIBSE CPD Nov 23.indb 22 02/11/2023 16:51