An air-source heat pump does not make heat by burning anything. It moves heat that already exists from one place to another. That one idea explains almost everything about how it works.
A fridge does the same job. It takes heat out of the food inside and pushes it out through the back. A heat pump does this on a bigger scale, for a whole building.
There is heat in cold air
It sounds odd, but even cold winter air holds heat. Air at 2°C feels cold to us, but it still contains plenty of heat energy. A heat pump is built to collect that energy and move it indoors.
Most modern units keep working well below freezing. They do become less efficient as the outside air gets colder, which is why good design matters.
The aerosol and the bike pump
Two everyday things show the science. When you spray an aerosol can, the can gets cold. When you pump up a bike tyre, the pump gets hot. Letting a gas expand cools it down. Squeezing a gas heats it up.
A heat pump uses both effects. It has a special fluid, called a refrigerant, sealed inside a loop of pipes. The refrigerant boils at a very low temperature, so even cold air can turn it from a liquid into a gas.
Here is the loop, step by step:
- Outside, a fan blows air over a coil of pipes. The cold refrigerant inside picks up heat from the air and turns into a gas.
- A compressor squeezes that gas. Like the bike pump, this makes it much hotter.
- The hot gas passes through a heat exchanger indoors. It hands its heat over to the water in your heating system, or straight to the air in the room. As it cools, it turns back into a liquid.
- The liquid passes through a valve that drops its pressure. Like the aerosol, this makes it very cold again, ready to collect more heat outside.
Why it uses so little electricity
The electricity in a heat pump runs the compressor and the fans. It is not turned straight into heat, the way it is in a kettle or a fan heater. It is used to move heat, and moving heat takes far less energy than making it.
Engineers measure this with the coefficient of performance, or COP. It is simply the heat you get out divided by the electricity you put in. A plain electric heater has a COP of 1: one unit of electricity in, one unit of heat out. A well-designed heat pump can reach up to about 5 in good conditions, so one unit of electricity gives up to five units of heat. In very cold weather the figure is lower.
If the electricity comes from your own solar panels, that heat costs very little to produce.
It can cool as well as heat
Many air-source heat pumps can run the loop in reverse. In summer they take heat out of the building and push it outside. That is exactly what an air conditioner does. One system can keep a building warm in winter and cool in summer.
What makes a good installation
A heat pump works best when it is matched to the building. That means working out how much heat the building loses, choosing the right size of unit, and putting the outdoor unit where it has good airflow and will not disturb anyone.
Heat pumps also like to run steadily at lower temperatures, rather than in short hot bursts like a gas boiler. Radiators, underfloor heating and controls all need to suit that. Get these things right and a heat pump is a simple, quiet and very efficient way to heat and cool a building.