Answer by Dale Blundell (Atkins)
The working temperatures of hot water heating systems with heat pumps systems are lower than conventional heating, typically 30°C to 55°C. Conventional systems will typically operate with water circuit temperatures of 80°C flow and 70°C. This means that heat pumps are ideal for use with under floor water heating, which is designed for operation at around 30°C.
The lower water temperatures mean that for the same heat output the radiators need to be typically 30% larger than standard (again this suits underfloor heating with large surface areas of heat emitters). The lower temperatures also mean that it is important that the house is well insulated and air tight with low draughts.
These technical requirements mean that heat pumps are particularly attractive in new build properties, less so in older style properties built to lower standards.
ASHPs can also be used to provide both heating and domestic hot water. It is important that where hot water is generated by any type of ASHP, it is stored (or periodically heated to) above 60°C to avoid legionella growth which can, if necessary, be achieved by using top-up heat from an alternative source. The efficiency of heat pumps falls as the output temperature increases, and so heating domestic hot water results in a lower COP than space heating.