Air is pulled over a series of coils by a finely balanced fan unit. The coils contain a liquid refrigerant which absorbs the heat from the air. The refrigerant boils at around -20°C so, even if the air outside has dropped below -10°C during very cold weather, it will still be warm enough to make the refrigerant boil.
The vaporised refrigerant is then compressed to create a hot gas, up to 100°C, which is used to heat the water contained in the thermal store via a heat exchanger.
The efficiency of an air source heat pump varies as the outside temperature changes. Overall, we expect most systems to generate 3kW of heat for every 1kW of electricity used, creating an average coefficient of performance (COP) of 3.