HEAT PUMPS | PIPEWORK AND RADIATORS Heat pumps will not have spare capacity to operate in very cold weather below the design outside air temperature The mean radiant temperature is calculated by multiplying the area of each surface in a room by its temperature, summing these products together, then dividing the result by the total surface area. Clearly, the use of radiator surface temperatures of less than 45C compared with the current 65C or more will produce a lower overall mean radiant temperature, and achieving the same comfort level will generally require a higher air temperature and, hence, radiator outputs. Procedure for radiator and pipework design Figure 3: A bypass circuit maintaining 5K T across the heat pump while allowing the radiators to operate at 10K T The solution to cold weather operation would appear to be a hybrid system for example, retaining a boiler for use below 2C and interlocking it so it cannot operate at the same time as the heat pump. Some gas condensing boilers have a calibrated temperature-setting control, which allows the user to adjust the flow temperature to a specific value. Typically, flow temperatures from 45C to 82C can be set on the dial, which allows the performance of an existing heating system at low temperatures to be evaluated before heat pump installation. This could give useful data when determining the extent of radiator upgrades required. Following a room-by-room heat-load calculation, radiators should initially be sized based on the flow and return temperatures for the selected heat pump. Radiators of the same length as the existing ones, but of a higher output type and/or greater height, should be chosen, with the aim of using the existing pipework. A check of the pressure drop and velocity in each pipe will show whether radiators can be operated at the heat pump T or whether the radiator T needs to be greater than the heat pump T, noting that it is only when the smallest pipe sizes are present that it may be necessary to replace pipework. All of the calculations required can be done using the radiator and pipework calculator, downloadable at bit.ly/CJApr22RPSC DAVID PALMER, is now retired. He was formerly a director of the Campbell Palmer Partnership Comfort The currently accepted measure of comfort is expressed by the operative temperature (To). CIBSE Guide B1 defines To as the average of the air temperature and the mean radiant temperature as in equation 2: T0 = 0.5Tr + 0.5Ta FLOOR MOUNTED LIFTING STATIONS DrainMajor Acknowledgements CHRIS HARVEY, Stelrad head of marketing UK & Ireland, for support, use of Stelrad data, and validation of radiator calculations RYAN KIRKWOOD, heat pump business development manager Baxi Heating, for important contributions to the article 0118 9821 555 Compli 400 Authorised by Jung Pumpen GmBH POWERED BY Compli 1000 Compli 2500 www.jung-pumps.co.uk www.pumptechnology.co.uk 50 April 2022 www.cibsejournal.com CIBSE Apr 22 pp48-50 Upgrading homes for heat pumps.indd 50 25/03/2022 14:35