The short answer
An air source heat pump (ASHP) is a heating system that extracts low-grade heat from the outside air and upgrades it to a useful temperature to warm your home and hot water. It works like a refrigerator running in reverse: a refrigerant absorbs heat from the air even when it is cold outside, is compressed to raise its temperature, and then releases that heat into your radiators, underfloor heating or a hot water cylinder. Because it moves existing heat rather than burning fuel, an ASHP delivers roughly 3 to 4 units of heat for every unit of electricity it uses — an efficiency expressed as its SCOP. Most UK installations are air-to-water units that connect to a wet central-heating system. They typically run at a lower flow temperature than a gas boiler, which is why heat pumps suit well-insulated homes and larger radiators.
Air source heat pumps are now the default low-carbon heating choice for most UK homes, and the type supported by the Boiler Upgrade Scheme grant. Understanding what they actually do — and what they don't — explains why they behave differently from the gas boiler they often replace.
Air source heat pump key facts
- Heat sourceOutside air
- Most common UK typeAir-to-water (connects to wet heating)
- Typical efficiency (SCOP)Around 3.0 to 4.0
- Typical flow temperatureAround 35 to 55 °C
- Grant supportBoiler Upgrade Scheme £7,500
How an air source heat pump works
An ASHP contains a sealed circuit holding a refrigerant — a fluid that boils at a very low temperature. The cycle has four stages:
- Evaporation: A fan draws outside air across an evaporator coil. Even cold air contains usable heat, which boils the refrigerant from a liquid into a gas.
- Compression: An electric compressor squeezes the gas, which sharply raises its temperature.
- Condensation: The hot gas passes through a heat exchanger, releasing its heat into the water that feeds your radiators, underfloor heating or hot water cylinder. As it gives up heat, it condenses back to a liquid.
- Expansion: The liquid passes through an expansion valve, its pressure drops, and it cools ready to absorb heat again.
The key point is that the electricity is used to move and concentrate heat, not to create it by burning fuel. That is why a heat pump can be more than 100% efficient in conventional terms — it returns several units of heat per unit of electricity consumed.
Air-to-water vs air-to-air
There are two broad categories of air source heat pump, and the distinction matters:
- Air-to-water: the most common UK type. It heats water that circulates through radiators or underfloor heating and is stored in a hot water cylinder. This is what people usually mean by "air source heat pump" and the type eligible for the Boiler Upgrade Scheme.
- Air-to-air: heats (or cools) air directly and distributes it through fans or ducts, similar to air conditioning. It does not heat domestic hot water and is not covered by the Boiler Upgrade Scheme.
An air-to-water unit consists of an outdoor unit (the fan, compressor and evaporator) and connects to indoor components such as a hot water cylinder and the existing wet heating system.
For UK homes the practical reason air-to-water dominates is that most properties already have a wet central-heating system — radiators fed by circulating hot water. An air-to-water heat pump slots into that familiar arrangement, reusing much of the existing pipework and many radiators, and it heats domestic hot water through a cylinder. Air-to-air, by contrast, is closer to the heating-and-cooling systems common in warmer climates: it blows warmed air into rooms and offers cooling in summer, but it cannot heat your taps and showers, so a separate hot water solution is still needed. That combination of needing a separate hot water source and not qualifying for the Boiler Upgrade Scheme is why air-to-air is rare as a whole-home heating choice in Britain, even though it can suit specific spaces such as a single room, an extension or a workshop.
Efficiency and how it compares to a gas boiler
A heat pump's seasonal efficiency is measured by its SCOP (Seasonal Coefficient of Performance) — the average heat output divided by electricity input across a typical year. A well-designed installation commonly achieves a SCOP of around 3.0 to 4.0, meaning 3 to 4 kWh of heat for each kWh of electricity. A modern gas boiler is typically rated at roughly 90% efficient, returning less than one unit of heat per unit of gas burned.
Because electricity costs more per kWh than gas in the UK, the higher efficiency is what closes — and can reverse — the running-cost gap. The lower the flow temperature a system runs at, the higher its SCOP tends to be, which is why correct radiator sizing and good insulation matter so much.
| Feature | Air source heat pump | Gas boiler |
|---|---|---|
| Heat source | Outside air | Burning mains gas |
| Typical efficiency | SCOP around 3.0–4.0 | Around 90% |
| Typical flow temperature | 35–55 °C | 60–80 °C |
| Carbon at point of use | None on site | CO2 from combustion |
Indicative comparison for guidance only. Sources: Energy Saving Trust; MCS.
What an ASHP needs to work well in a UK home
A heat pump is only as good as the system it is plumbed into. Because it delivers heat at a lower temperature than a gas boiler, the rest of the heating system has to be able to release that gentler heat into your rooms. Three things decide whether it performs:
- Emitter sizing: radiators must be large enough to warm each room at the lower flow temperature, or underfloor heating used where practical. The MCS heat loss survey identifies which radiators are adequate and which need upsizing.
- Insulation: the better insulated the home, the less heat it loses, the lower the flow temperature can run, and the higher the SCOP. Loft and cavity wall insulation often pay back twice — lower bills and a more efficient pump.
- Controls and weather compensation: a heat pump runs most efficiently when it heats steadily and adjusts its output to the outside temperature, rather than blasting on and off. Weather compensation nudges the flow temperature up only as far as the cold demands.
This is why a heat pump is best thought of as a whole-system upgrade, not a like-for-like box swap. Run gently and continuously across a well-insulated home with correctly sized emitters, it is comfortable and economical. Forced to run hot through small radiators in a draughty home, it will cost more and disappoint. The design work done before installation matters as much as the unit itself.
Frequently asked questions
Does an air source heat pump work in winter?
Yes. ASHPs are designed to extract heat from air down to well below freezing, and certified units in the UK are rated to operate at low ambient temperatures. Efficiency falls as it gets colder, so a correctly sized system and a hot water cylinder help meet peak demand on the coldest days. Most UK homes can be heated by an ASHP through a normal British winter.
Is an air source heat pump noisy?
Modern units are relatively quiet, producing a low hum from the fan and compressor. Installations under permitted development have a noise limit to meet, assessed at the nearest neighbouring property. Sensible siting away from bedrooms and boundaries keeps noise unobtrusive for most households.
Can an air source heat pump heat my hot water?
Yes, an air-to-water heat pump heats domestic hot water and stores it in a hot water cylinder, replacing the role of a combi or system boiler. Because the cylinder stores heated water, the pump can heat it efficiently during off-peak periods. A periodic higher-temperature cycle is used to keep the cylinder safe from bacteria.
Sources & further reading
- Energy Saving Trust — Air source heat pumps
- MCS — Heat pumps for consumers
- gov.uk — Boiler Upgrade Scheme
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.