Air source heat pumps explained - Which? (2024)

An air source heat pump is an alternative way to heat your home. It will enable you to generate your own renewable heat and potentially save you money on your energy bills in the long run.

They deliver heat at lower temperatures than gas and oil boilers, so you'll need to run them for longer periods to heat your home to a comfortable temperature.

A well-insulated home is essential – otherwise, the heat generated by the pump escapes more easily and you may find the temperature doesn't get up to a level you're used to.

A heat pump could save more on your heating bills, but this will depend on whether you're replacing an expensive system – such as electric storage heaters, oil, LPG (liquefied petroleum gas) or coal – along with how efficiently your heat pump is running, and whether or not you're on a heat pump tariff.

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Air source heat pump advantages and disadvantages

Here are some of the key advantages and disadvantages:


  • Heat pumps are much more environmentally friendly than gas boilers. They run on electricity, and more and more of our electricity is being generated from renewable sources, such as solar and wind power. Renewable energy's share of overall generation was 47.3% in 2023, up from 41.5% in 2022, so the electricity used to power heat pumps is becoming cleaner.
  • Heat pumps are highly efficient because they generate more energy than they consume. For every one unit of energy it uses, a typical heat pump produces between 2.5 and 4 units of heat – or even 4.5 units – making heat pumps 250-400% – or 450% – efficient. Efficiency varies depending on system design, system sizing, heat loss and the weather, so it's essential that your heat pump is running efficiently to get these high returns. Modern condensing boilers are labelled between 92-94% efficient, making them much less efficient that heat pumps. Note that boilers also need to be set up for maximum efficiency to get these outputs. Boilers older than 2005, when new regulations came in, are significantly less efficient.
  • You could save money on heating compared to using a gas boiler if you have a high quality heat pump, set up well, that you use effectively, and if you're on a heat pump tariff.


  • You'll need enough space outside your home for the external condenser unit, and you may need indoor space for a hot water cylinder.
  • To get the most from your heat pump, you may need work done to your home to insulate it, and you may need to install new, larger radiators or an underfloor heating system. You can still install a heat pump without this, but it won't be running as efficiently and you may be paying for heat that is lost. That said, it's wise to insulate your home anyway, even if you're sticking with a gas boiler, as this will make your home more comfortable and save you money on heating bills.
  • Installing a heat pump requires a lot of effort because heat-loss calculations and a careful design of your heating system are crucial to ensure you're happy with your comfort and your bills.
  • If your heat pump is unsuitable or set up poorly, you could find yourself spending a lot more on energy than before, as electricity costs more than gas.

Note that many of these cons can be mitigated with correct setup, and are mainly just points to be aware of.

How does an air source heat pump work?

An air source heat pump takes heat from the outside air and boosts it to a higher temperature using a compressor. It then transfers the heat to the heating system in your home.

They work a bit like refrigerators in reverse.

Air source heat pumps explained - Which? (2)
  1. The air source heat pump absorbs heat from the outside air into a liquid refrigerant at a low temperature.
  2. Using electricity, the pump compresses the liquid to increase its temperature. It then condenses back into a liquid to release its stored heat.
  3. Heat is sent to your radiators or underfloor heating. The remainder can be stored in your hot water cylinder.
  4. You can use your stored hot water for showers, baths and taps.

The pump uses electricity to run, but it should use less electrical energy than the heat it produces. This makes it an energy efficient way to warm your home.

Air source heat pumps work even if the temperature is well below zero.

Ground source heat pumps also harness natural heat and increase the temperature to warm your home. Find out how ground source heat pumps work.

What is an air source heat pump?

An air source heat pump is a low-carbon way of heating your home. They absorb latent heat from the outside air and use it to increase the temperature inside your home.

Air source heat pumps look similar to air-conditioning units. Their size depends on how much heat they'll need to generate for your home – the more heat, the bigger the heat pump.

There are two main types of air source heat pumps: air-to-water and air-to-air. They work in different ways and are compatible with different types of heating systems.

Air-to-water heat pumps

Air-to-water heat pumps take heat from the outside air and feed it into your wet central heating system.

They're most suitable for homes with larger radiators or water underfloor heating because the heat they produce is cooler than that from a conventional gas or oil boiler. To be most effective, they need a large surface area to release the heat.

It is more straightforward to incorporate larger radiators or underfloor heating for a heat pump while you're extending your home or in a new-build property. It can also cost less than retrofitting underfloor heating later on.

Air source heat pumps explained - Which? (3)

Air-to-air heat pumps

Air-to-air heat pumps take heat from the outside air and feed it into your home through fans. You need a warm-air circulation system to move the heat around your home.

These systems cannot produce hot water, so you'll need a separate immersion heater or other water-heating system.

In the summer, an air-to-air heat pump can operate in reverse. In other words, you can use it like an air-conditioning unit to provide cool air for your home.

If you're looking for an air conditioner, check our air conditioner reviews – just be aware that they can be energy-guzzlers and you should aim to pick one that uses energy efficiently.

Air source heat pump installation

Air source heat pumps explained - Which? (4)

Air source heat pumps are usually positioned outdoors at the side or back of a property. They need plenty of space around them for air to circulate.

You will either have:

  • a monoblock system – most common in the UK when the external unit can be placed on or next to an outside wall. The external unit generates hot water and sends it into your property. If you only want it to provide heating (via radiators/underfloor heating), that's all you need. If you want the heat pump to also provide your hot water, you need an indoor hot-water storage cylinder.
  • a split system – used when there's quite a distance between the external unit and house. This consists of an external unit and an internal heat exchanger about the size of a small boiler. Again, if it's just for heating, that's all you need, but if you want it to provide your hot water, you'll also need a hot-water cylinder.

You don't need a hot-water storage cylinder if you choose to install a heat pump for heating only (to heat your radiators/underfloor heating) and use a separate method for hot water (such as an electric shower, instant hot water taps or instantaneous hot-water unit)

Air source heat pumps are less disruptive to install than ground source heat pumps, as they do not require any digging in your garden.

Speak to your home insurance provider to check if your policy will cover the changes to your heating system.

If you're getting an air source heat pump, it's important to make sure that your home is well-insulated so that it can retain the heat. Underfloor heating or larger radiators are often installed alongside heat pumps to disperse the heat better.

Your installer should tell you how to use the controls for your heat pump to help you use it most effectively. You will probably need to heat your home for more hours but at a lower temperature.

When your system is completed, you should get a Commissioning Certificate from the installer. You should also get a Microgeneration Certification Scheme (MCS) installation certificate once the system has been registered (the installer must do this within 10 days). You'll need this to qualify for most funding schemes.

Are air source heat pumps efficient?

A typical heat pump should generate 2.5 to 4 units of heat for each unit of electricity it uses, in effect making heat pumps 250% to 400% efficient. The Coefficient of Performance can be even higher than this, though it can also be lower, especially in the depths of winter.

The Seasonal Coefficient of Performance (SCOP) tells you the average efficiency of your heat pump across the whole heating season. Your installer should calculate this and give you an estimate of SCOP before installing your heat pump. This gives you a realistic view of costs and efficiency that accounts for the accumulative warm, mild and cold days we face during the autumn and winter.

To get the best from your heat pump, you'll need to know how to use it most effectively. Often, you'll need to set your heating to come on for longer than with a traditional system. Your installer should show you how to control your heat pump system.

You should also have your heat pump serviced every two to three years. Check that any grills are free of leaves and debris on a regular basis, and follow any other maintenance checks advised by your installer.

An air source heat pump system can help to lower your carbon footprint as it uses a renewable, natural source of heat – air. How much CO2 you'll save depends on the fuel you are replacing. For example, the figure will be higher if you are replacing coal or an oil boiler, rather than natural gas.

A heat pump needs a power source – usually electricity – to power the heat pump, so there will still be some resulting CO2 emissions, but much less than from using a gas boiler.

Find out how to calculate your heat pump running costs to see what efficiency you need to save money versus running a typical gas boiler.

Where can I buy an air source heat pump?

You'll need an MCS-certified installer who can apply for government grants on your behalf.

If possible, get multiple quotes for a heat pump installation because the costs may differ and recommendations may vary between assessors.

Energy companies offering heat pumps and installations include:

  • British Gas offers a survey, installation and aftercare. Its 'Warm Home Promise' means it will give you your money back if your heat pump doesn't reach an agreed temperature for your home. In practice, this just means fitting a system that's the right size, but it doesn't guarantee a specific level of efficiency.
  • EDF Energy partners with a company called CB Heating, which installs Daikin heat pumps with a five-year warranty.
  • E.ON Energy is a Which? Trusted Trader offering Daikin, Mitsubishi and Samsung heat pump installations with seven-year guarantees.
  • ScottishPower offers Daikin, Vaillant and Mitsubishi heat pumps with two years of annual servicing from Domestic & General.
  • Octopus Energy offers survey, quote, installation and aftercare. It offers a five-year warranty.

If you prefer to use a private installer, MCS hosts a database of MCS-certified contractors.

These installers agree to follow a code of conduct known as RECC (Renewable Energy Consumer Code). This means they should not use high-pressure selling techniques, misrepresent the benefits of a heat pump in your home or offer you better rates if you do marketing work for them.

Air source heat pumps explained - Which? (2024)
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