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How does an air source heat pump work?
Published and last updated 15 October 2026
An air source heat pump (ASHP) works like a fridge in reverse: a fan draws outside air across a coil of refrigerant, the refrigerant absorbs the air's warmth, a compressor raises its temperature, and a heat exchanger passes that heat into the water for your radiators and hot water cylinder. The Energy Saving Trust describes heat pumps as more than three times more efficient than a gas or oil boiler.
What are the four stages of the heat pump cycle?
Every air source heat pump repeats the same four-stage refrigerant cycle: evaporation, compression, condensation and expansion.
- Evaporation — outdoor air passes over the evaporator coil and the cold refrigerant absorbs its heat, turning into a gas.
- Compression — the compressor squeezes the gas, which raises its temperature.
- Condensation — the hot gas gives up its heat to your heating water through a heat exchanger and turns back into a liquid.
- Expansion — an expansion valve drops the refrigerant's pressure and temperature so the cycle can start again.
What are the main parts of an air source heat pump?
The main parts are the outdoor unit (fan, evaporator coil and compressor), the heat exchanger, the expansion valve, the refrigerant, the controls and, for air-to-water systems, a hot water cylinder.
- Fan and evaporator coil
- Compressor
- Heat exchanger (condenser)
- Expansion valve
- Refrigerant
- Controls and thermostat
- Hot water cylinder
- Buffer tank (only where the design needs one)
Where does the heat actually come from?
Most of the heat comes from the outside air; electricity only runs the fan, compressor and pumps that move it.
COP is heat out divided by electricity in at a moment; SCOP is the same measured across a year, so a SCOP of 3 means three units of heat for each unit of electricity.
How does a heat pump heat your hot water?
An air-to-water heat pump heats water stored in a cylinder rather than heating it on demand like a combi boiler.
The Energy Saving Trust says a standard system typically needs a cupboard of at least 80cm by 80cm for the cylinder, and systems periodically heat stored water to a higher temperature as a hygiene measure.
What's the difference between monobloc and split heat pumps?
A monobloc heat pump keeps every component in one outdoor unit, while a split system divides them between an outdoor and an indoor unit.
| Type | Layout | Pros | Cons |
|---|---|---|---|
| Monobloc | All components outdoors | Cheaper and quicker to install, less indoor space | Slightly less efficient |
| Split | Outdoor and indoor units | Can be slightly more efficient | Takes more indoor space and costs more |
Source: Energy Saving Trust.
How is an air-to-air heat pump different?
An air-to-air heat pump uses the same cycle but blows warm air through indoor units instead of heating water, so it doesn't provide hot water.
Does an air source heat pump work in cold weather?
Yes. The refrigerant boils at very low temperatures, so the unit keeps extracting heat when it's below freezing, and it runs short defrost cycles to clear ice from the outdoor coil.
Why does a heat pump run differently from a boiler?
A heat pump heats water to a lower temperature and runs for longer at a steady output, which is more efficient than short bursts of high heat.
Weather compensation adjusts flow temperature to outdoor conditions.
How is this different from a gas boiler?
A gas boiler creates heat by burning fuel, while a heat pump moves heat that's already in the outside air.
Frequently asked questions
Is an air source heat pump the same as air conditioning?
They use the same refrigerant cycle, but an air-to-water heat pump heats water for radiators while air conditioning cools air; air-to-air heat pumps can do both.
Is a heat pump the same as a central heating pump?
No. A central heating pump is the small circulating pump that moves water round your radiators; an air source heat pump is a complete heating system.
What makes the noise?
The fan and compressor in the outdoor unit make a low hum while running.
Does it need electricity to work?
Yes, to run the compressor, fan and pumps, but it delivers several times more heat than the electricity it uses.