How does a heat pump work?
Short answer
A heat pump moves heat rather than creating it directly. Refrigerant is compressed to raise its temperature, then expanded so it becomes colder than the outside air and can absorb heat. The cycle repeats, moving heat from outside into your radiators, underfloor heating and, in many systems, your hot water cylinder.
Last checked 24 September 2026 · 2 sources
Is a heat pump basically a fridge in reverse?
Yes. That is the easiest way to picture it.
A fridge moves heat from inside the cabinet to the room. That is why the back of a fridge feels warm.
A home heat pump uses the same basic vapour-compression cycle, but its useful output is the heat delivered into your home.
How can it take heat from cold air?
Cold air still contains thermal energy.
Inside the outdoor unit, refrigerant is allowed to expand. This drops its pressure and temperature. Once the refrigerant is colder than the outdoor air, heat can flow into it.
The compressor then squeezes the refrigerant. Compressing it raises its temperature. That higher-temperature heat can then be transferred into the water circulating around your heating system.
Why can a heat pump be more than 100% efficient?
Because it is moving heat, not turning electricity directly into heat.
A direct electric heater can turn roughly one unit of electricity into one unit of heat. A heat pump can often deliver several units of heat for each unit of electricity it uses.
You may see this described as COP, or coefficient of performance. A COP of 3 means three units of heat are delivered for each unit of electricity used at that moment.
SCOP means seasonal coefficient of performance. It is the average efficiency across a heating season and is usually more useful when comparing real-world performance.
Does the refrigerant go into my radiators?
Usually not in an air-to-water heat pump.
The refrigerant stays inside the sealed refrigeration circuit. It transfers heat into the water that then circulates through your radiators or underfloor heating.
What should I do next?
If you are comparing systems, ask installers for the expected seasonal efficiency at the proposed flow temperature, not just the headline efficiency of the heat pump unit.
Sources
- Source interview with Heat Geek installer, vapour-compression explanation and fridge comparison.
- Energy Saving Trust, heat pump guidance, checked 24 September 2026.
Last checked: 24 September 2026