For ASHPs, the COP gets worse as outdoor temperatures drop, as the ASHP then uses more electricity to keep the house warm. Properly installed ground source heat pumps don't have this problem as the temperature underground is almost constant. Factors such as low winter temperatures and the demand for domestic hot water mean that the high COPs quoted in brochures may be rarely achieved. When comparing quoted COPs, check what source and delivery temperatures they’re based on, and if hot water is included. Ask installers for figures that reflect winter air temperatures, such as the following figures for the COPs of two Worcester Bosch air-to-water heat pumps at various outdoor and delivery temperatures:
|Temperature (Inlet)||Temperature (Delivery)||Heat Pump CoP (7kW)||Heat Pump CoP (9.5kW)|
Source: Worcester Bosch
A German study (Frauenhofer Institute) found that ASHPs in new buildings achieved an average COP of 3.0, while those added to existing buildings had an average COP of 2.6 (very few of these had underfloor heating).
An Energy Saving Trust field trial of 83 heat pumps in the UK found a wide variance in performance - only a few reached a COP of 3 or more. The average COP for air source heat pumps was 2.1, while the average for ground source systems was 2.3. Many were early installations, and as installers gain experience, performance should improve. To avoid a low COP, ensure that a home is well-insulated, has a low-temperature heating system and good heating controls, and that the ground loop or air-source unit has been adequately sized.
Some ASHPs are air-to-air rather than air-to-water. If looking at this as an option, bear in mind that warm air heating systems are uncommon in the UK and hard to install into existing buildings because of the ductwork required. Make sure you’ve included the full costs of this equipment.