Air Source Heat Pumps

Air Source Heat Pumps

Air source heat pumps use electricity to capture energy from the air to heat your home and hot water. Air source heat pumps look like air conditioning units and are generally installed at ground level outside your house.

How do Air Source Heat Pumps work?

Air source heat pumps extract heat from the air via a heat exchanger. Even though the air is not normally as warm as you would like your home it is usually warm enough to cause a special liquid refrigerant to evaporate and turn into a gas.

This gas is then be sent through a compressor, increasing its pressure and causing its temperature to rise. The higher temperature gas is then passed through a heat exchanger and the heat is transferred to into a central heating or hot water system.

The heated gas falls in temperature as the heat is transferred and subsequently returns to a liquid state. The cycle repeats until the required temperature is reached. Reversible heat pumps can provide heating in the winter and cooling in the summer.

how do air source heat pumps work

Air Source Heat Pump efficiency

A well-installed air source heat pump is supposed to deliver three kilowatt hours (kWh) of heat for each kWh of electricity it consumes. This ratio is called the coefficient of performance (CoP). The higher the CoP, the more efficient the system is – some guides and manufacturers quote CoP figures as efficiency figures meaning that an air source heat pump with a CoP of 3 is listed as “300% efficient” but don’t be fooled, these are not perpetual motion machines! As with ground source heat pumps, the CoP of an air source heat pump can vary considerably depending on how well it is installed, how well sized it is in comparison to your home, how many radiators you have, how hot you keep them and also with variations in the weather. In winter an air source heat pump may deliver far less than a CoP of 3. (Tip: consider installing smart radiator valves to easily shut off the heating in unoccupied rooms.)

A CoP of 3 or above is seen as good because it means that the cost of running the heat pump should be cheaper than a gas boiler, for the equivalent heat output since gas is currently about a third of the price of electricity. is a brilliant, open source project via which people share and compare heat pump performance data, so you can see a variety of installations and compare detailed statistics to see how performance can vary.


The main advantage of air source heat pumps is that they can be zero carbon, when powered by renewable electricity. So, if your electricity is supplied from solar PV, or via a guaranteed renewable energy tariff, replacing your gas boiler with an air source heat pump can reduce your heating and hot water emissions to zero. You may also be able to reduce the running costs of an air source heat pump by switching to one of the new smart tariffs.

Air source heat pumps also qualify for the Boiler Upgrade Scheme, via which the UK government offers grants of £5,000.

When combined with solar PV, air source heat pumps make a lot of sense because the renewable energy generated by the solar PV in the day can be used by the heat pump making you largely self-sufficient, presuming your PV system is big enough to power the heat pump.

Air source heat pumps don’t require the ground space of ground source heat pumps.


Some people have had terrible experiences with air source heat pumps. They are not cheap to install. One man claims he spent £27,000 having an air source heat pump installed and is facing a £7,000 energy bill every year.

Air source heat pumps must be correctly sized and installed, to ensure they run efficiently and deliver a CoP of 3 and above. A badly installed or sized system which delivers a CoP of less than 3 will leave you paying more than you would have paid when burning gas.

Installing an air source heat pump will definitely increase your electricity bill – and since electricity is currently more than 3 times the price of gas in the UK any increase in electricity consumption can have a big impact on bills. It is amazing how many people forget to factor in the extra costs of electricity when considering a heat pump – and forget to monitor how much more electricity they are consuming after having one installed. Always take a baseline before installing a heat pump so you can calculate its true efficiency after it is installed.

Air source heat pumps require a hot water tank so may not be suitable for flats and smaller homes. Installation works can be considerable, depending on how much pipework and plumbing needs replacing.

Air Source Heat Pump costs

The total cost of an air source heat pump can vary considerably depending on the size of the house, how well insulated it is, how many radiators and how much pipework needs replacing. We estimate that the installed cost for a 3 bed home should be between £7,000 to £13,000 although air source heat pumps also qualify for a £5,000 grant from the Boiler Upgrade Scheme which will help reduce the costs.

Nesta have developed an Air Source Heat Pump cost calculator which uses artificial intelligence to look at installations that have happened over the last few years and estimate the probable total cost in your home.

Running costs for an air source heat pump can also vary dramatically, depending on the size of your home and the CoP your system delivers. We estimate that the running costs for a system which is delivering a CoP of 3, for a 3 bed home should be around £1,127/year, which is a slight saving in comparison to heating your home and hot water with gas.

Is an Air Source Heat Pump worth it?

If your motivation is to reduce your carbon emissions to zero then yes, an air source heat pump is definitely worth it, providing your home is suitable. However, the cost and disturbance of installation mean we do not recommend installing an air source heat pump unless you’re building a new home, or need to replace an existing boiler.

Check out the other alternatives to gas boilers in our energy guide.