How does an air source heat pump work?
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A heat pump works like a fridge in reverse. Instead of moving heat from inside the fridge cabinet to the room outside, it moves heat from outside air into your home — even when outside temperatures are below 0°C.
The outdoor unit contains a refrigerant that evaporates at very low temperatures. Even cold outdoor air has enough energy to turn the refrigerant from liquid to gas.
The gas refrigerant is compressed by an electrically-driven compressor, which dramatically raises its temperature — similar to how a bicycle pump gets warm when you inflate a tyre.
This hot refrigerant passes through a heat exchanger (condenser) which transfers the heat into your home's water circuit, heating your radiators or underfloor heating.
The now-cooled refrigerant is expanded back to a low pressure gas, ready to absorb heat from outside again — and the cycle continues.
The key measure of efficiency is the COP (Coefficient of Performance). A COP of 3 means for every 1 kWh of electricity used, you get 3 kWh of heat. In practice, UK air source heat pumps average a COP of 2.5–3.5.
Heat pumps work best with low-temperature heating systems (35–45°C flow temperature) like underfloor heating or large, oversized radiators. This is why they are less efficient when fitted to old radiator systems designed for 70°C+.
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