The essentials in 30 seconds
A heat pump takes heat that is naturally in the air or ground, even when it feels cold outside, and raises its temperature for your central heating and hot water. It does it with a refrigeration cycle, the same one a fridge uses but in reverse: a refrigerant absorbs outside heat and evaporates, an electric compressor squeezes it into a hot gas, the gas releases its heat into a water circuit for your radiators or underfloor heating, and an expansion valve cools it again to repeat the cycle. For every unit of electricity, a heat pump gives about 3 to 4 units of heat. It works harder in winter, so look at the seasonal efficiency (SPF) your installer calculates, not only the datasheet CoP. In the UK an air source heat pump costs around £12,000 and a ground source one around £15,000; in England and Wales the Boiler Upgrade Scheme gives £7,500 towards one.
- Air source heat pumps are the most common in UK homes.
- You usually need space for a hot water cylinder.
- Some radiators may need upgrading.
How do heat pumps work, step by step?
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01
Evaporation
Heat from the air or ground reaches a heat exchanger with a liquid refrigerant. The refrigerant absorbs it and evaporates into a low-pressure, low-temperature gas.
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02
Compression
An electrically powered compressor raises the gas's pressure, which raises its temperature.
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03
Condensation
The hot gas passes its heat to a water circuit in a second heat exchanger. The warm water flows to your radiators or underfloor heating, and the refrigerant cools to a high-pressure liquid.
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04
Expansion
An expansion valve lowers the pressure, cooling the refrigerant below the outside temperature so it can absorb heat again. The cycle starts over.
How do air source and ground source heat pumps work?
| Air source | Ground source | |
|---|---|---|
| Where the heat comes from | Outside air, blown over the heat exchanger by fans | The ground, through a water and antifreeze mix (brine) in a ground loop or borehole |
| Space | An outdoor unit about the size of two wheelie bins side by side | A large garden for the ground loop, or boreholes |
| Efficiency | Very efficient | Tends to be more efficient |
| Typical UK cost | Around £12,000 | Around £15,000 |
| Suits | Most types of home; the most common in the UK | Homes with a lot of outdoor space |
For flats and park homes, the Energy Saving Trust notes that an air-to-air heat pump, which heats with fans instead of radiators, may suit better.
How efficient are heat pumps?
Because they move heat instead of generating it, heat pumps deliver about three to four units of heat for every unit of electricity (300 to 400%). An A-rated gas or oil boiler, by comparison, converts about 90% of its fuel into heat.
| Figure | What it means |
|---|---|
| CoP (Coefficient of Performance) | Heat out per unit of electricity under test conditions; a CoP of 3.0 means three units of heat per unit of electricity |
| SCoP or SPF (seasonal) | Efficiency averaged over the year, which is a better guide to running costs |
How do heat pumps work in winter?
There is still usable heat in the air and ground when it feels cold, so a heat pump keeps working in winter, but it has to work harder when it is colder outside. That is why the datasheet CoP alone does not tell you the running cost. Your installer must calculate the Seasonal Performance Factor (SPF) for your home, based on local average temperatures and your radiator sizes, and should share it with you before starting work. Correctly sized radiators, the right controls and a suitable tariff all affect running costs.
Is a heat pump electrical?
Yes. The compressor runs on electricity, and that is the main energy a heat pump uses. On standard tariffs, where electricity costs nearly four times as much as gas, the Energy Saving Trust says a heat pump can cost slightly more to run than a new gas or oil boiler. On a heat pump time-of-use tariff, it can cost less to run than a gas boiler. You save more if you can disconnect the gas and stop paying the gas standing charge.
What a heat pump costs in the UK
- Air source: around £12,000, depending on heat pump size, property size, new build or existing home and radiator upgrades.
- Ground source: around £15,000, and they are considerably more expensive to install than air source.
- Grants: in England and Wales, £7,500 through the Boiler Upgrade Scheme. In Scotland, interest-free loans or grants may be available. In Northern Ireland there is no specific heat pump grant at present.
How heat pumps work: frequently asked questions
How do heat pumps work in simple terms?
They take heat from the outside air or ground, raise its temperature with an electric compressor and pass it to your heating system, using a refrigeration cycle like a fridge in reverse.
How do air source heat pumps work?
Fans blow outside air over a heat exchanger, where a refrigerant absorbs its heat. A compressor raises the temperature and the heat goes into the water for your radiators or underfloor heating.
How do ground source heat pumps work?
A water and antifreeze mix circulates through pipes buried in the garden or in a borehole, collecting heat from the ground for the same refrigeration cycle.
Do heat pumps work in winter?
Yes. There is still heat in cold air and ground, but the heat pump works harder, so check the seasonal efficiency (SPF) your installer calculates.
How efficient is a heat pump?
About 3 to 4 units of heat for every unit of electricity, compared with about 90% for an A-rated boiler.
Does a heat pump use electricity?
Yes, to run the compressor. A heat pump time-of-use tariff can make it cheaper to run than a gas boiler.
Sources
Independent energy advice, checked on 2 November 2026 (02/11/2026).
- Energy Saving Trust · Heat pumps: how they work, costs and savings ↗Heat in air and ground even when cold; refrigeration cycle in four steps (evaporation, compression, condensation, expansion); fridge in reverse; fans for air source and brine in ground loops or boreholes; 3 to 4 units of heat per unit of electricity versus 90% for an A-rated boiler; CoP, SCoP and SPF; harder work in winter; SPF calculated by the installer; air vs ground source; air-to-air for flats; hot water cylinder and radiators; around £12,000 air source and £15,000 ground source. Read in the browser.
- Energy Saving Trust · Air source heat pumps: costs, savings and benefits ↗How air source heat pumps work; most common in UK homes; cost factors; £7,500 Boiler Upgrade Scheme in England and Wales, Scotland loans and grants, no specific grant in Northern Ireland; running costs depend on radiators, tariff and controls; electricity nearly four times the price of gas; time-of-use tariffs; gas standing charge. Read in the browser.
Thinking about a heat pump?
Ask the installer for the Seasonal Performance Factor calculated for your home and the radiator upgrades it assumes.