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How do heat pumps work?

Heat pumps take low temperature heat from the air, ground, or water outside your home, concentrate it, raising the temperature, and it can then be used to heat your home and provide hot water.

Heat pumps are powered by electricity and the heat that they take from the air, ground or water is really just heat from the sun - so in a way they capture solar heat and transfer it into your home.

Heat pumps are typically 250% efficient or more (efficiencies of 300%+ are common), which means that for every kW of electrical energy they use, you get 2.5 kW of heat, or more. This efficiency is called the COP of the heat pump. COP stands for the Coefficient of Performance. A 250% efficient heat pump would have a COP of 2.5.

So, how do they do it?

It helps to first take a quick look at liquids turning to gasses (and vice versa), and then at something more familiar - a fridge.

To turn from a liquid to a gas takes energy. The atoms in a liquid are more "sticky" than in a gas - you have to break that "stickiness", and that takes effort/energy.

Boiling pan of water

You know when you are boiling a pan of water? It doesn't suddenly just go "poof!" and all turn to steam and disappear the minute it's at 100ºC does it. No, it takes time for it to boil away - quite a lot of time. All that time your hob is busy still putting more heat into the water - where's it all going? Well it's being used up turning liquid to gas!

Man sweating infront of a fan - sweat cools us down because it takes energy to turn a liquid to a gas

When you're hot you sweat and that evaporates, turning from a liquid to a gas. It takes heat to turn a liquid to a gas - that heat is taken away from your skin - and so you cool down, which is why you sweat. So heat is being extracted - in this case from your skin - and transferred away from you.

This process also works in reverse - so when the gas changes to a liquid state heat is given out.

A heat pump is like a fridge

In a fridge there is a refrigerant inside a loop of pipe. On one side the refrigerant is at a high pressure, and so is a liquid. This bit gets hot and so gives out heat. In the other part it is at a lower pressure, and this bit gets cold and so taken in heat.

Diagram showing the refrigeration cycle - which is how heat pumps work

A pump (called a compressor) pumps it around this loop - halfway in the loop is a pressure valve which helps by keeping the pressure on one side high, and low on the other. The high pressure part where the refrigerant is a liquid is called the Evaporator. The low pressure part where it is a gas is called the Condenser.

The evaporator absorbs heat - so in your fridge it takes the heat away from the inside of the fridge which keeps your food cool and your ice cubes frozen.

The condenser gives heat out - in the case of your fridge you may have noticed that the outside gets warm?

In the case of an air source heat pump the evaporator takes heat away from the air outside, and the condensor provides hot water to heating for your home.

So if you wanted to heat your house up with a fridge you could cut a fridge sized hole in an outside wall, push your fridge, with the door facing out, so it's poking just through the hole. Open the door and turn it on.

House with a fridge in the wall open to the outside to illustrate how a heat pump works

Your fridge will now cool down the outside world, and transfer that heat to warm up your house - just like a heat pump.

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