Air source (air-to-water)
Takes heat from outdoor air and warms water for radiators, underfloor heating and a hot water cylinder. The common choice for UK homes.
Heat pumps
Heat Pumps for UK Homes
A heat pump moves heat that already exists outside into your home, rather than burning fuel to create it. That one difference explains almost everything else about how they are designed, installed and run.
Grant support through the Boiler Upgrade Scheme applies to England and Wales. Whether a particular property qualifies is assessed at application.
What a heat pump is, and how it works
The short version: it is a fridge running in reverse, at the scale of a house.
A refrigerator takes heat out of the food inside it and dumps that heat into your kitchen. A heat pump does the same thing in the other direction: it takes heat out of the outside air or the ground and delivers it into your home.
It does this with a refrigerant that boils at a very low temperature. Outside, the refrigerant absorbs heat and evaporates. A compressor then squeezes that vapour, which raises its temperature sharply. Indoors, the hot vapour gives up its heat to your heating system and condenses back to a liquid, and the cycle repeats.
Because the electricity is used to move heat rather than to create it, a heat pump delivers several units of heat for each unit of electricity it uses. That ratio is the coefficient of performance, and it is the single number that matters most for running cost.
The catch is that the ratio depends on how hard the system has to work. The bigger the gap between the outside temperature and the temperature of the water it produces, the harder it works. That is why heat pump systems are designed around low flow temperatures, and why the design work matters more than the brand of the box.
The three types used in UK homes
They are not variations on a theme. Air-to-air in particular is a genuinely different proposition from the other two.
Ground source
Takes heat from buried ground loops or boreholes. More efficient and quieter, but the groundworks are a project in themselves.
Air-to-air
Delivers warm air directly to rooms. No radiators, no cylinder — and no hot water, which is the part people miss.
| Type | Heat source | What it heats | Suits | What you see |
|---|---|---|---|---|
| Air-to-water | Outdoor air | Radiators, underfloor heating, hot water cylinder | Most homes | Outdoor unit with clear airflow |
| Ground source | Ground loops or boreholes | Radiators, underfloor heating, hot water cylinder | Properties with land or drilling access | Trenches or boreholes; nothing visible afterwards |
| Air-to-air | Outdoor air | Warm air to rooms — no radiators, no cylinder | Homes without a wet heating system | Outdoor unit plus indoor units in each room served |
What a property needs
Most UK homes can be heated by a heat pump. What varies is how much has to change to make it work well.
Radiators and the rest of the heating system
A heat pump runs at a lower water temperature than a boiler, so each radiator gives off less heat than it used to. The fix is more surface area: larger radiators, or higher-output ones, in the rooms where the calculation shows a shortfall. In many homes it is a handful of rooms rather than all of them.
Existing pipework is usually fine. Microbore pipework and heavily sludged systems need more attention, which a survey will pick up.
Insulation and heat loss
Reducing heat loss reduces the size of heat pump you need and the electricity it uses. Loft insulation and draught-proofing are cheap and almost always worth doing first. Solid wall insulation is a much bigger decision, and a good design can often work around a solid-walled property by sizing the emitters accordingly.
"Your home must be perfectly insulated before you can have a heat pump" is not true, and it puts people off unnecessarily.
Hot water
Air-to-water and ground source systems heat water for a cylinder. If the property has a combi boiler and no cylinder, space has to be found — an airing cupboard, a loft, or occasionally a purpose-built enclosure. In flats and small terraces this is often the binding constraint.
Where the equipment goes, and noise
An air source outdoor unit needs clear airflow and sensible positioning relative to windows and neighbours. Siting is governed by permitted development conditions in England and Wales, and your installer should tell you which route applies rather than leaving you to find out later. Ground source systems have nothing outdoors once the work is complete.
Electrical supply
Most homes need nothing unusual. Some need a consumer unit change, and occasionally the local network operator needs to be notified or involved. This is checked at survey.
The question worth asking any installer
"What flow temperature have you designed this system to run at, and can I see the room-by-room heat loss calculation?" A confident answer to that tells you more than any brochure. There is more on this in our guide to whether a home is suitable for a heat pump.
What installation involves
From first survey to living with the system. The design stage is the one that decides how well it performs.
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Step 1: Survey and heat loss calculation
Room by room, the installer works out how much heat the property loses at a design outdoor temperature. This sets the size of the heat pump and the size of every radiator — it is the foundation of everything that follows.
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Step 2: System design
The design fixes the flow temperature the system will run at, which emitters need changing, where the outdoor unit and cylinder go, and what the pipework and electrics need. Ask to see it. A design that shows its working is the best predictor of a system that performs.
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Step 3: Quotation and decisions
You get a scope and a price. This is also the point at which grant eligibility is assessed, and where the installer submits a scheme application if one applies.
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Step 4: Installation and commissioning
Typically several days for an air source system, longer where groundworks are involved. Commissioning sets the controls and the weather compensation curve, and this is where a good installation is separated from a merely finished one.
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Step 5: Living with it
A heat pump runs gently for longer rather than in short bursts. Expect a short settling-in period while the controls are tuned to how you actually use the house.
Grant support
The Boiler Upgrade Scheme applies to England and Wales and contributes towards eligible installations.
Air-to-water and ground source heat pumps currently attract £7,500 where the installation qualifies, and air-to-air systems £2,500 in residential properties. A higher value is available for properties off the mains gas grid that are replacing oil or LPG heating, for a limited period.
The scheme is installer-led: an MCS-certified installer applies on the property owner's behalf and the value is deducted from the installation cost. Full detail is on our heat pump grants page, and the criteria on the eligibility page.
Frequently asked questions
Do heat pumps work in cold weather?
Yes. They extract heat from air or ground that is colder than the inside of your home, which is what a refrigeration cycle is for. Efficiency does fall as it gets colder outside, which is why a system is sized against a design outdoor temperature rather than an average one.
Do I need underfloor heating?
No. What matters is that the emitters can give off enough heat at a lower water temperature, so some radiators are usually upsized. Underfloor heating suits a heat pump very well, but it is an advantage rather than a requirement.
Are heat pumps noisy?
An air source outdoor unit has a fan and is audible close up, comparable to a quiet conversation at a short distance. Position matters, and there are siting rules that a competent installer will work to. Ground source systems have nothing outdoors at all once the work is finished.
Will I need a hot water cylinder?
For an air-to-water or ground source system, yes — they heat water for storage rather than instantly. If the property currently has a combi boiler and no cylinder, finding space for one is one of the first practical questions to settle.
How long do heat pumps last?
Manufacturers generally expect a service life in the region of two decades for the heat pump itself, with an annual check. A ground array lasts considerably longer than the heat pump connected to it.
Is a grant available towards the cost?
In England and Wales, the Boiler Upgrade Scheme may contribute towards an eligible installation. Eligibility depends on the property, the existing heating system and the technology, and is confirmed when the installer applies.
Thinking about a heat pump for your home?
Tell us about the property and what heats it now, and we will come back to you about the options.