Air source heat pumps explained

An air source heat pump is an alternative way to heat your home. It will help you reduce your impact on the environment and potentially save you money on your energy bills in the long run.

Heat pumps 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. 

They work most efficiently in well-insulated homes – 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 lower your heating bills, but this will depend on whether you're replacing a system that's more expensive to run – 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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What is an air source heat pump? 

An air source heat pump absorbs heat from the outside air and uses 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 (also know as hydronic heat pumps) take heat from the outside air and feed it into your wet central heating system. This type of heat pump can be a direct replacement for your gas or oil boiler.

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. 

If you currently have a wet central heating system, an air-to-water heat pump will be easier to fit into your exisiting set-up than other types. They are more expensive than air-to-air heat pumps, but you can get a government grant (the Boiler Upgrade Scheme) for £7,500 towards your costs.  

Air-to-air heat pumps

Air-to-air heat pumps take heat from the outside air and feed it into your home through fans. This type of heat pump is not a direct replacement for a gas or oil boiler, as it doesn't heat radiators. It is an alternative to other types of electric heating (storage heaters or panel heaters), and could be a good choice for flats where an air-to-water system is not feasible or where you are hoping to heat (and cool) a small space.

To heat multiple rooms, you need a warm-air circulation system to move the heat around your home through ducts. Otherwise, you need a separate unit for each room (similar to the way individual air conditioning units are often installed for each room in hotter countries).

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.

Air-to-air heat pumps are cheaper than air-to-water ones, but you will need to create a new circulation system in your home as they don't use a wet central heating system (radiators and pipes). They are not covered by the government's grant scheme.

If you're looking for a stand alone air conditioner, check our air conditioner reviews.

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 on the same principle as a fridge or freezer - moving heat from one place to another. 

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. In a wet system, this heat warms up water to be 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.
5. In a dry system, this heat is circulated as warm air through ducts and vents. Air-to-air heat pumps can circulate either hot or cold air.

Heat pumps use electricity to run, but they are more than 100% efficient. This means they produce more heat energy in kWh than they use in electricity. This makes it an energy efficient way to warm your home. By contrast, gas boilers consume more energy than they put out into your home. Modern condensing gas boilers are labelled between 92-94% efficient. Boilers installed before 2005, when new regulations came in, are significantly less efficient.

Air source heat pumps work even if the temperature is below freezing (zero degrees Celsius) because there is heat energy present in air at any temperature above absolute zero (-273 degrees Celsius).

Ground source heat pumps also harness natural heat and increase the temperature to warm your home. 

Find out how ground source heat pumps work.

Air source heat pump advantages and disadvantages

When thinking about switching to a heat pump, there are a number of pros and cons to consider.

Pros

• Heat pumps are more environmentally friendly than gas boilers because they emit no carbon dioxide at the point of use. They run on electricity - if you get yours from the National Grid, more of this electricity is generated using renewable sources - such as solar and wind power - than ever before. Renewable energy's share of overall generation was 47.3% in 2023, up from 41.5% in 2022. 
• Heat pumps are highly efficient. They generate more heat than the electricity they consume. For every unit of energy it uses, a typical heat pump produces between 2.5 and 4 units of heat, making heat pumps around 250-400% efficient. Efficiency varies depending on system design, system sizing, heat loss and the weather, so it's essential that your heat pump is set up well to get these high returns. 
• You could save money on heating compared to using a gas boiler or other heating system if you install a suitably-sized heat pump that is set up and used efficiently, and you buy cheap electricity on a heat pump tariff. 

Cons

• You need suitable space outside your home for the external condenser unit, and you may need indoor space for a hot water cylinder. This is often not an issue for larger homes with a garden, but could be inconvenient or even impossible in a smaller property. Some types can be mounted on an external wall rather than on the ground, if you don't have outside ground space.
• You may need to replace existing pipework and/or radiators in your current wet central heating system to make sure it can deliver enough warmth with an air-to-water heat pump. This is to compensate for the lower flow temperatures supplied by a heat pump.
• You will probably need to boost your home's insulation to get the most from your heat pump. You can install a heat pump without doing this, but you might find you need a larger model, and it won't be running as efficiently. 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. 
• It's more effort than installing a new boiler. Installing a heat pump for the first time requires a lot more effort and planning than simply replacing a gas boiler with a new one. You'll need a careful design of your heating system, based on accurate heat-loss calculations, to achieve your desired comfort levels and minimise your bills. The installation and associated works may take a number of days and could be somewhat disruptive.
• Heat pumps must be set up correctly, which creates room for error. If your heat pump is unsuitable or set up incorrectly, you could find yourself spending a lot more on energy bills. You'll need to be willing to learn how to operate your heating system, as it can be different from a gas boiler.

Note that many of these downsides are one-time-only when you install your first heat pump, or can be simply addressed by getting the correct set-up and good advice from a competent installer.

Air source heat pump installation

Air source heat pumps are usually positioned outdoors on the ground at the side or back of a property. Where this is not possible, they can also be located at the front, or mounted on a wall, balcony or flat roof. 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 you generate. 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 a longer period 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 around 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 grilles are free of leaves and debris on a regular basis, and follow any other maintenance checks advised by your installer.

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.

Are air source heat pumps good in winter?

All heating systems have to work harder to keep your home warm in freezing weather. The colder the outside air, the more quickly heat is lost from your home through the walls, windows and roof, so the boiler has more work to do to continually replace that lost heat.

Air source heat pumps are designed to work at low temperatures - down to minus 10C, which is sufficient for most parts of the UK - but they use more electricity to extract sufficient heat from the air. In very cold weather, efficiency may drop from around 300-400% to perhaps 200%; however, they are still more efficient than a gas boiler.

Typical efficiency (COP) varies at different temperatures. For example:

Heat pumps are surprisingly popular in cold countries. The top four European nations with the most heat pumps per person – Norway, Sweden, Finland, and Estonia – experience much colder winters than the UK. It's common to see heat pump units covered in a thick layer of snow that are still working perfectly well.

Heat pumps have an automatic defrost mode that should kick in to prevent the system from icing up. However, a broken sensor, low levels of refrigerant, or blocked vents can lead to a frozen system, so it's important to keep up with the maintenance schedule.

If you live in one of the colder parts of the country, you can get a heat pump that's designed to work down to minus 25 degrees. Alternatively, consider a ground source heat pump; they are more efficient in winter because the ground temperature stays reliably warmer than the air temperature.

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. They also offer a discounted rate on electricity your heat pump uses, for a fixed period.
• EDF Energy partners with a company called CB Heating, which installs Daikin heat pumps with a five-year warranty. They also offer a Heat Pump Tracker tariff with discounted rates. 
• E.ON Energy is a Which? Trusted Trader offering Daikin, Mitsubishi and Samsung heat pump installations with seven-year manufacturers' warranties. 
• Good Energy is a Which? Trusted Trader offering Midea heat pumps with a 10-year warranty and remote monitoring, which it claims are '20% more efficient than industry standards on average'.
• Octopus Energy supplies survey, quote, installation and aftercare, including to non-customers. It offers an 8-year warranty on its own-brand heat pumps (5 years warranty on other brands), and also has a range of tariffs suited to heat pump owners.
• Ovo has partnered with installer network Heat Geek to offer heat pump installation. It also offers a 'Heat Pump Plus add-on' giving you cheaper electricity rates for the energy your heat pump uses.
• Scottish Power offers Daikin, Vaillant and Mitsubishi heat pumps with two years of annual servicing from Domestic & General. It also offers an off-peak heat pump tariff.

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.

You can also search for heat pump installers through Which? Trusted Traders to find a company that we've vetted and hold to strict standards.