How do air source heat pumps work?
An air source heat pump collects heat from its surroundings outside with a fan drawing air through a radiator (like a car radiator) which has been cooled by the system refrigerant being pumped through it. This is why an air source heat pump can still produce heat when the outside temperature is below freezing, as long as the temperature in the radiator is lower than the surrounding air it will absorb heat. In fact it will still work when the temperature falls to -15˚C.
It’s all due to the refrigerant cycle which has been unused in refrigerators for many years. The cold refrigerant gas absorbs heat, the gas is then compressed which changes from gas to liquid. The heat that was absorbed by the gas is concentrated in the liquid which is now hot enough to heat the water in the radiators. So the hot refrigerant liquid is pumped alongside the water from the central heating in a heat exchanger. The heat is transferred from the liquid refrigerant into the central heating water and then the process starts again.
To make the radiator in the heat pump cold enough to absorb more heat from the air, the refrigerant liquid having given some of it’s heat to the central heating water goes into a low pressure zone within the system where it is allowed to expand, because it is now at a lower pressure the liquid turns into gas which cools it down, the gas is pumped around the radiator surrounding the heat pump which makes it colder than the air again so it can absorb more heat.
The heat pump is transferring heat from the air to the central heating system by compressing and expanding the refrigerant, changing it from liquid to gas, cooling it down so it can absorb heat and then from gas to liquid, warming it up so it can give its heat to the central heating system.