How An Air Source Heat Pump Works

What is an Air Source Heat Pump and how does it work?

An Air Source Heat Pump (ASHP) is a very efficient way of heating your home as it can provide more energy than it consumes in operation. Whilst that might initially sound like magic and witchcraft, the way it does it is both very clever and very simple… and yet strangely familiar.

The reason why an ASHP will be familiar to you is because it works by using the process of refrigeration, it is essentially an inside out fridge. You may well have noticed that whilst a fridge is cold on the inside, it has a load of pipework on the back of it that can get very warm. This pipework is known at the condenser coil, and an ASHP uses the heat produced it it’s condenser coil to heat your home.

But how does it actually do that?

It all comes down the expansion and compression of liquid and gas. A really good example of this is deodorant body spray! You will have noticed that even if the can is warm to the touch, when you spray your body, the spray is cold. This is because the small amount of liquid sprayed from the can rapidly expands into a gas filling a much larger volume. That gas still has the same amount of heat energy as the liquid but it’s now having to fill a much larger volume, which means the temperature drops. If you were to contain that gas in a sealed network of pipe and allowed the gas to increase in temperature by warming to its environment, and then compressed the gas back into a liquid, the opposite process would happen. When the warm gas is compressed it turns into a hot liquid.

The diagram below shows how this process works on a heat pump.

how a heat pump works

So the heat isn’t produced by electricity then?

Whilst the heat in your home is produced by the refrigeration process rather than by electricity, you will still need electricity to power the pump itself. But this is the clever bit, it only takes around 1kW of electricity to power the pump to make around 3kW of heat. This is what is known as the Coefficient of Performance (CoP) and it’s how the efficiency of ASHP is measured.

However, because an ASHP gets its energy from the air outside, when the outside air temperature drops, so can the efficiency of the heat pump. This leads to a separate measure known as the Seasonal Coefficient of Performance (SCoP). The bigger the difference between outside air temperature and the temperature that you want to achieve inside, the lower the efficiency. This is why carry out very specific calculations around what’s known as the “Heat Load” of your home to ensure that the most efficient size of ASHP is specified.

You can find out more about the two different brands of ASHP we use here: