How Does an Air Source Heat Pump Actually Work?

Lots of us struggle with the idea of how a heat pump can take heat from the outside where it is cold and use it to warm our homes. Especially when we read that air source heat pumps can function at below zero temperatures and still keep our houses warm, right? How does that work?


All air above absolute zero contains energy in the form of heat. This energy is used to heat a refrigerant (a liquid with a very low boiling point) in an evaporator. The refrigerant, absorbs heat from the air and evaporates (turns into a gas). It then goes through a compressor which increases its pressure and temperature. From the compressor the gaseous refrigerant goes through a condenser. In the condenser the refrigerant condenses from gas back to liquid, giving off heat. In an air to water system this heat is transferred to water and can then be used to run radiators, underfloor heating etc.

Having released its energy and returned to a liquid state, the refrigerant goes through an expansion valve which lowers its pressure again and therefore lowers its temperature. The refrigerant then returns to the evaporator where it can again absorb heat from the outside. Both the evaporator and the condenser are heat exchangers. The evaporator is in contact with the outside air and absorbs heat from it. The condenser is in contact with water and transfers heat to it.

If you use an air source heat pump to heat your house and hot-water you will have a box-like machine outside your property. The most obvious component is a large, quiet fan system which sucks in air and blows back the cooler air which has transferred some of its heat to the refrigerant. You will also see a double pipe entering and leaving the building. One is warm as it is carrying the heated refrigerant and the other is cool as it returns the cool refrigerant back to the pump. You will also notice the electricity supply to the pump.

A well installed air source heat pump can provide year round heat and hot water, saving you significant sums on your heating and energy bills. Into the bargain, you will reduce your carbon footprint.

A good system will be individually designed to suit your house or business. With costs starting from £4,000, savings on your energy bills and the RHI, it is an attractive alternative heating option.

Find more in our introduction to air source heat pumps and our guides to the Renewable Heat Incentive and the Domestic Renewable Heat Incentive. If you would like to find out how an air source heat pump would work for your property and the costs and issues involved, we suggest you contact 3 or more accredited installers. You can find accredited air source installers serving your area in our directory.

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