HEAT PUMPS | HIGH TEMPERATURE n Are there hydronic inefficiencies such as exposed, poorly insulated pipework or temperature dilution that can be addressed? n Has the building been extended/reduced/ zoned or had alternative heating systems installed in localised areas? n Can the true building load requirements in summer, winter and transient months be measured or calculated with a degree of accuracy? n Are any bounding spatial constraints yielding enough to allow for new plant to be installed? n Does budget allow for 100% of the required heating power to be via ASHPs? n Are electrical capacities sufficient? n Are there factors to offset the potential higher running costs, such as PV? Design information for dated buildings is often limited to a hand-drawn schematic on the plantroom wall. To add to the confusion, many will have seen a dated building run, at some point, on one boiler out of three during winter, with no complaints. Improving our understanding of the building profile can be done through installing items such as ultrasonic heat meters, undertaking a full heat-loss calculation (if budget and time allow) and using known data, such as gas-meter readings. Extrapolation of live data or interpolation of fragmented historic data help piece together the jigsaw, for a greater insight into the true thermal profile. The goal here is to understand what might be changeable, what cant change, and the risks. For example: n Flow temperatures may be reduced by fixing hydronic inefficiencies. n Bracketing of the heating system may reduce the requirement to run all circuits at 80C or 82C all year, improving running costs (see below, Bracketing). Figure 1: Typical two-boiler reverse return header system n Spatial challenges can be solved by sizing real requirements through measuring and calculation. This may take months to complete, and can be further complicated by seasonal conditions. Ideally, this would involve at least a years worth of data, with any subsequent installation planned for warmer months. Bracketing Bracketing involves consolidating the heating system into frames of known and weighted data. For example, if the survey data shows a sizable constant temperature (CT) circuit serving an air handling plant exclusively, the decision may be taken to bracket this out of the overall heating system. By bracketing this circuit and serving it directly from its own heat pump plant, we are now able to change the tempering or reheat coils to suit a 55C flow temperature (or lower). This decision alone could increase the heat pump efficiency by up to 150% from a design temperature of around 80C. The same principle can be applied to variable temperature (VT) circuits when the CT circuit is unable to deviate from the current design flow temperature. VT circuit bracketing can yield massive efficiency rewards, as the weather compensation can be undertaken at the plant without the use of mixing valves. With direct weather compensation on HT ASHPs, the flow temperature could range from 35C-80C. If heat losses mitigations have been carried out then, potentially, emitters may be changed when and if possible, to allow a more aggressive reduction in flow temperature. The proportion of the year when the HT ASHPs must remain at 80C flow may be offset, in terms of net efficiency, by the period of time that flow temperatures are not required at 80C via direct weather compensation. The weighted aspect of bracketing involves understanding the split in capacity required for each circuit. If VT equates to 80% of the overall load requirement, then addressing that in isolation, with CT remaining on 80C flow, may impact the Figure 2: Blended mediumtemperature/high-temperature heat pump solution 34 March 2024 www.cibsejournal.com CIBSE March 24 pp33-34, 36 Baxi Heat Pump.indd 34 23/02/2024 13:48