CIBSE GUIDANCE | HEAT-PUMP INSTALLATION 1 0.9 0.8 Diversity factor 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 20 40 60 80 100 120 140 160 180 200 Number of dwellings Figure 3: DS 439 diversity curve for DHW demand of an indicative development for this period of time, so the design must be able to mitigate this loss of capacity. The applications manual explains: The difference between gross and integrated capacity of a heat pump Sizing heat-pump arrays to mitigate the impact of defrost cycles Sizing buffer vessels and thermal stores to overcome the effect of defrost cycles. All about temperature In new-build residential developments with high-performance fabric, domestic hot water (DHW) represents an increasing proportion of a buildings energy use. The provision of DHW also drives the output temperature of the heat pump and often dictates the peak output of a heat pump or communal system. A recently published guidance note from CIBSE consolidates the legislation and guidance around DHW temperatures and delivery times in instantaneous systems (such as heat interface units). Key findings are that: Generating instantaneous hot water at a temperature of 50C satisfies the requirements to reduce the risk of legionella growth and minimise the risk of scalding Delivering instantaneous hot water to the kitchen tap at a minimum of 45C within 45 seconds of opening the tap to full flowrate demonstrates an acceptable service level for users and sets a requirement that also limits water use. This unified guidance on DHW temperatures is key to unlocking the full efficiency of any heat pump that serves an instantaneous DHW system. Correctly sizing a heat-pump system In contrast to fossil-fuel boilers, the capacity of a heat pump must be closely matched with the demand it serves. Residential loads are highly variable, throughout the year and across any given day, making this a challenging requirement to meet. If undersized, the system may fail to heat rooms or DHW to the required temperatures. If a secondary heat source is available (an immersion heater, for example) these may run more than intended, driving up energy consumption, emissions and running costs. Conversely, if fixed-speed heat pumps are oversized, they will short-cycle at part load, impacting system efficiency. Most inverter- A case study of a Wilmott Dixon project features in AM16 driven units will also see a reduction in efficiency when operating at part load, but not to the same extent as fixed-speed units. The higher marginal capital cost and space taken by an oversized heat pump underlines the importance of accurately determining the required capacity. The Applications Manual reiterates the importance of correctly accounting for diversity, particularly of DHW demands, in centralised systems. The updated CP1 (2020) provides an invaluable reference to an English translation of the Danish DS 439 standard, which readily allows designers to exploit diversity values as low as 2-3% of the connected load (see Figure 3). The guidance also provides information on how to size buffer vessels and thermal storage, which can provide: System stability, management of defrost, peak lopping, heat sharing and resilience AM16 Heat-pump installations for multi-unit residential buildings will be released shortly at www.cibse.org/knowledge and will be a valuable resource for building designers and operators, as well as heat-pump installers, guiding practitioners in the successful application of this evolving technology to the residential market. CJ JOSHUA BIRD is a senior engineer at Arup Acknowledgements CIBSE would like to acknowledge the support of the NHBC Foundation in the production of this publication. 44 August 2021 www.cibsejournal.com CIBSE Aug 21 pp42-44 Heat pump guide.indd 44 23/07/2021 12:47