What Is an Air Source Heat Pump and How Does It Work?

An air source heat pump is a unit that sits outside your home and takes heat from the outside air, then uses it to heat your radiators or underfloor heating and your hot water. It replaces a gas, oil or LPG boiler, or electric storage heaters.

It works like a fridge in reverse. A liquid called a refrigerant runs through the outdoor unit and absorbs heat from the air, even when it's well below freezing outside. A compressor then squeezes the refrigerant, which raises its temperature, and that heat is passed into the water in your heating system. The refrigerant cools, expands and goes round again.

Because it moves heat rather than making it by burning fuel, a heat pump gives out much more energy than it uses. A well-designed system typically delivers three to four units of heat for every unit of electricity it uses across the year. That figure is called the SCOP, the seasonal coefficient of performance.

There are two main types. Air-to-water heat pumps, the most common in the UK, heat water for radiators, underfloor heating and a hot water cylinder. Air-to-air heat pumps blow warm air into rooms, a bit like air conditioning, and can also cool in summer, but they don't heat water.

Air-to-water heat pumps installed by an MCS certified installer like Casa Renewables can qualify for the £7,500 Boiler Upgrade Scheme grant in England and Wales, which comes straight off your invoice. Air-to-air systems are not covered by the scheme.

To work well, a heat pump has to be sized to your home's heat loss, and the radiators have to be big enough to heat each room at the lower water temperatures heat pumps run at. That's why every installation starts with a room-by-room survey. Book a free survey and we'll tell you whether a heat pump suits your home, and what it would cost.