Air Source Heat Pumps Explained

An air source heat pump is a heating system that extracts heat from the outside air to heat your home and your water¹. It works like a fridge in reverse: it absorbs heat from its source, the outside atmosphere, and uses a compressor to raise that heat to a temperature warm enough for radiators, underfloor heating or a hot water tank². The unit sits outside your house, and it can get heat from the air even when the temperature is as low as minus 15 degrees Celsius³.

The reason householders hear so much about them is that they replace burning gas or oil with electricity, and they use that electricity efficiently: instead of converting every unit of electricity into one unit of heat, they move existing heat from the air into the home. In the UK the main financial support is the Boiler Upgrade Scheme, which gives £7,500 off the cost and installation of an air source heat pump⁴. Installation prices themselves are installer-quoted, and they vary with the size of the house, the emitters and the work involved.

Air source heat pumps explained

An air source heat pump operates by absorbing heat from the outside air to provide heating⁸. That sounds implausible to many people, because British winter air does not feel like a source of warmth, but heat is energy, and there is energy in air at any temperature above absolute zero. The pump's job is to collect that low-grade energy and concentrate it.

The process runs in a refrigeration cycle. The outdoor unit contains a heat exchanger through which refrigerant passes: the refrigerant absorbs heat from the air, and the compressor then squeezes it to a higher temperature so it can be handed over to the heating system inside. As MCS puts it, the machines "work like a fridge in reverse. They absorb heat from their source, the outside atmosphere, and transfer it at low temperature to the pump compressor"². A fridge moves heat out of its cabinet and dumps it in your kitchen; a heat pump moves heat out of the garden and dumps it in your living room.

The heat that is delivered depends on the setup: air-to-water heat pumps source their heat from the air outside and distribute it by warming your radiators, underfloor heating, or hot water tank, depending on your setup⁹. Because the cycle is driven by electricity, the pump does not run without power, and that is the central dependence it creates: a household that moves from a gas boiler to a heat pump swaps a dependence on gas for a dependence on electricity. What it gains is the ability to heat the home from ambient air, with no fuel deliveries, no flue and no on-site combustion.

Air to air versus air to water systems

Air source heat pumps are classified as either air-to-air or air-to-water depending on whether the heat distribution system in the building uses air or water³. This is not a minor technical distinction: it decides what the system can heat, and it is the first question to settle before looking at products or prices.

An air-to-water system distributes heat via your wet central heating system³. In official terms, it "transfers heat drawn from surrounding air, to water in a wet central heating system"¹⁰. That means it feeds the radiators or underfloor pipes you already have, or new ones sized for the job, and it can also heat your water as well as your home². For most UK houses with existing wet heating, this is the configuration that replaces the boiler directly.

An air-to-air system produces warm air which is circulated by fans to heat your property³. It behaves more like air conditioning that can heat as well as cool, and it has one significant limitation: air-to-air heat pumps "deliver heat through a fan or warm-air circulation system, but these do not produce hot water", so a separate system is needed for water heating¹¹. A household choosing air-to-air therefore keeps, or installs, another means of heating water, whether an immersion heater, a hot water heat pump or something else.

Both types absorb heat from the outside or surrounding air and transfer that into useable heat in the home for space or water heating¹². The choice between them is covered in more detail on our pages on air-to-water heat pumps and air-to-air heat pumps, and the grant rules treat them differently too, as set out below.

Typical UK efficiency of about 300% and understanding COP

A heat pump does not convert electricity into heat one-for-one; it uses electricity to move heat, and it moves more heat than the electricity it consumes. The standard measure is the coefficient of performance, or COP. As Which explains, "a heat pump with an efficiency rating of COP 3.0 can produce three times more heat energy than the electrical energy it consumes"¹¹. A COP of 3.0 is the same thing as an efficiency of 300%: for every unit of electricity in, three units of heat come out.

That ratio is why heat pumps are described in percentages that look impossible next to a gas boiler, which cannot exceed 100% because it only converts fuel into heat¹¹. The comparison is not like-for-like in kind, but it is like-for-like in effect: the useful question for a householder is how much heat arrives in the home per unit of energy paid for. A COP of 3.0 means each unit of electricity buys three units of heat.

COP is a snapshot, measured at a particular outdoor temperature and flow temperature. It falls when the air outside is colder and when the water sent to the radiators is hotter, because the compressor has to work across a bigger temperature gap. That is why real-world performance over a whole season is usually discussed in seasonal measures such as SCOP, which we cover separately on heat pump efficiency: COP, SCOP and SPF. What COP tells you at the point of choosing a system is the order of magnitude: a well-installed air source heat pump delivers several units of heat per unit of electricity across a heating season, and the quality of the installation, the emitter design and the controls all move that number up or down.

Price and the Boiler Upgrade Scheme

The headline support figure is straightforward: the Boiler Upgrade Scheme gives £7,500 off the cost and installation of an air source heat pump⁴. The scheme covers £7,500 towards the cost and installation of ground and air source heat pumps, including water source heat pumps, backed by £2.7bn in funding to 2030¹³. The air source grant was increased from £5,000 to £7,500 in October 2023¹⁴.

The total installed price of a system is a separate matter, and it is installer-quoted: it depends on the size of the heat pump, the state of your radiators or underfloor heating, the hot water cylinder and the electrical work. There is no single published national price, so treat any figure a household sees as a quote for a specific house, not a market price. What the grant does is deduct a fixed £7,500 from that quote, and the scheme's own guidance sets out the voucher values and how long they last.

The voucher periods matter in practice: vouchers for air-to-water heat pumps, air-to-air heat pumps and biomass boilers are valid for 3 months, while ground source systems get 6 months because the ground works take longer¹⁵. Official statistics confirm the same periods: AWHP, AAHP and biomass boiler vouchers are valid for 3 months¹⁶. A household that lets a voucher expire has to reapply, so the installation needs to be booked and scheduled before the application, not after. The details of the scheme, including eligibility, are covered on our Boiler Upgrade Scheme and heat pump cost pages.

When running costs beat a gas boiler and when they do not

Whether a heat pump is cheaper to run than a gas boiler depends on three things: the price ratio between electricity and gas, the efficiency of the installed system, and the tariff the household is on. The upside is real. Which reports that "a household could save as much as 50% and more on running costs with a heat pump compared to a gas boiler", but that saving requires high quality installation, high efficiency and a heat pump tariff¹¹. All three conditions have to hold; remove one and the saving shrinks.

Government guidance makes the same point from the other direction: while running costs for heat pumps are typically lower when compared to those of traditional gas boilers, "if a system is" poorly installed or run at high flow temperatures, the advantage can be lost¹⁷. Electricity costs more per unit than gas, so a heat pump has to deliver substantially more heat per unit of electricity than a boiler delivers per unit of gas simply to break even. A COP of 3.0 does that comfortably when the price ratio is below three to one; a poorly performing system may not.

The tariff condition is worth dwelling on, because it is the one a householder controls after installation. Dedicated heat pump tariffs, and time-of-use tariffs that reward running the system when electricity is cheaper, change the arithmetic significantly, and the 50% figure assumes one is in place¹¹. We cover the comparison in detail on heat pump vs gas boiler running costs and heat pump running costs. The honest summary is that a heat pump can be markedly cheaper to run than a gas boiler, and that this outcome is earned by the installation quality and the tariff, not guaranteed by the technology alone.

Is your home suitable? Emitters, insulation and sizing

A heat pump is not a like-for-like swap for a boiler, because it prefers to deliver heat at lower water temperatures. The heat it produces goes into "warming your radiators, underfloor heating, or hot water tank, depending on your setup"⁹, and the setup is where suitability is decided. Lower water temperatures mean each emitter delivers less heat, so the total emitter area, or the water temperature, has to rise to compensate.

Underfloor heating suits the technology well because a large floor area can deliver plenty of heat at low water temperature. Existing radiators can work, but some may need to be larger, or the system may need to run slightly hotter, with a corresponding cost in efficiency. This is why a proper heat loss calculation comes before any quote: the installer measures how much heat each room loses and sizes both the heat pump and the emitters to match. We cover the emitter question in full on radiators and emitters for heat pumps.

Insulation sits behind all of this. A house that loses heat quickly needs a bigger heat pump, bigger emitters, or both, and the economics worsen at each step. A house with good insulation needs less of everything, and the same heat pump performs better in it. Neither fact makes insulation a legal precondition, but it is the single biggest lever on whether an air source heat pump is a comfortable, economical system or a strained one. Sizing and suitability are covered in depth on heat pump sizing and which homes suit a heat pump.

Where to put the outdoor unit and how loud it is

The outdoor unit is the part of the system your neighbours will notice, and the rules around it are specific. For a unit installed as permitted development, noise levels for an air source heat pump on its own must stay at or below 42 decibels (dB) from a metre distance away from any habitable room⁶. In the UK, the legal noise limit for heat pumps is 42 decibels¹⁸. The main sound sources in ASHPs are the fan and the compressor⁷, which is why noise depends heavily on the model and on how hard it is working.

Where a unit exceeds the MCS noise standards and planning permission is needed instead, a stricter condition can apply: noise levels emitted from the air source heat pump must not exceed 37 dB LAeq at any time, as measured on the boundary of the site with any neighbouring residential property¹⁹. The difference between the two limits is the difference between the permitted development route and the planning application route, and it is why the MCS 020 noise assessment matters to the installation.

Siting rules are equally concrete. All parts of the air source heat pump must be at least one metre from the property boundary, and the outdoor unit should be no bigger than 0.6 cubic metres⁶. Beyond the hard limits, guidance asks for care: the air source heat pump, so far as practical, should be sited to minimise its effect on the external appearance of the building and the amenity of the area²¹. If a planning application is needed, it should include details of location, design and appearance, noise and vibration, and efficiency of the unit⁸.

For perspective, government expert guidance notes that heat pumps are located outside and are typically quieter than gas boilers²². The noise question, including how the measurement is actually taken, is covered on how noisy is a heat pump?, and siting options on where a heat pump outdoor unit can go. Research for government noise guidance has examined possible revisions to permitted development guidance and regulations, drawing on local planning authorities, ASHP manufacturers, ASHP installers, and industry bodies²³, so the rules in this area may change.

When planning permission is required

In most cases, installing an air source heat pump at a house is permitted development, meaning no planning application is needed, provided the unit is located within your property and it meets permitted development size and location limits²⁴. Permitted development rights allow air source heat pumps to be installed on detached houses or blocks of flats without an application, subject to conditions²⁰. That is the default position, and it covers most installations that meet the noise and siting rules above.

The exceptions are worth knowing in advance:

• A second unit. Only the first installation of an air source heat pump would be permitted development; additional air source heat pumps at the same property require planning permission⁶. More precisely, planning permission is needed where there is an existing air source heat pump on a building or within the gardens or grounds of a dwelling house which is not detached, or two existing units on a detached house²⁵.
• Roof siting. Planning permission is needed if the unit is on a pitched roof or less than 1 metre from the edge of a flat roof²⁵.
• Listed buildings and monuments. If your property is listed, you will usually need listed building consent before installing an air source heat pump, even if planning permission is not required²⁴; for a listed building, both planning permission and listed building consent are needed²⁵, and you cannot install in a listed building or scheduled monument without planning permission²⁶.
• Flats and shared buildings. Permitted development rules for air source heat pumps are more restricted for flats and shared buildings, and planning permission is more likely to be required²⁴.
• Non-domestic buildings. Permitted development rights for small scale renewables do not extend to non-domestic buildings such as offices, schools and industrial premises, and planning permission may be required²⁰.
• Conservation areas and prominent positions. Planning permission may be required where the installation does not meet permitted development rules, where there are conservation area restrictions, where the unit would sit in a prominent position such as the front of the property, or where there would be an unacceptable impact on neighbours through noise or visual impact²⁴.

The nations differ in detail. In Scotland, Class 6H permits the installation, alteration or replacement of an air source heat pump on a dwelling or within the curtilage of a dwelling, and the pump must be used only for the purpose of providing domestic heating or hot water²⁷. In Northern Ireland, the permitted development legislation for air source heat pumps carries the same one metre boundary condition: development is not permitted if any part of the air source heat pump would be installed within 1 metre of the boundary of the curtilage of another dwelling house²⁹. Wales has its own regime, with a research report on ASHP noise and permitted development rights informing how the rules are applied⁷. We cover each nation separately on heat pump planning permission in England, Scotland, Wales and Northern Ireland.

For contrast, ground source and water source heat pumps are usually considered to be permitted development on domestic premises, not needing an application for planning permission, because the visible plant is minimal or absent³⁰. That difference is one of the practical arguments for ground source heat pumps in constrained sites.

Fitting, certification and maintenance

The certification requirement sits at the centre of the installation process. To benefit from permitted development, the air source heat pump must comply with the Microgeneration Certification Scheme Planning Standards (MCS 020) or an equivalent standard³¹. MCS 020 is the noise and installation standard, and compliance with it is what allows a unit to be installed without a planning application in the ordinary case. The installer should be MCS certified, and the Microgeneration Certification Scheme's consumer pages exist specifically to help householders find certified installers of air source heat pumps².

The Boiler Upgrade Scheme adds its own requirements on top: the installer applies for the voucher, the installation must meet the scheme's standards, and the voucher clock of three months for air source systems runs from issue⁵. That sequence, from survey and heat loss calculation through to commissioning and handover, is covered step by step on heat pump installation: the process from survey to handover.

Once the system is in, maintenance is light but not zero. It is a good idea to schedule a yearly check-up with a professional². An annual service typically covers the refrigerant circuit, airflow, filters and controls, and it is the evidence base for any warranty claim, a point we develop on heat pump servicing and maintenance. If something goes wrong with the installation, the route is defined: raise it with the installer first, and if the complaint is not resolved, you can approach the MCS Administrator, who will advise how to make a complaint about the MCS installer³². Ofgem also publishes dispute resolution guidance for the schemes it administers³².

How long they last and what ownership really gives you

Two figures are given for how long a unit lasts, and they differ: on average, an air source heat pump lasts up to 20 years², while a Welsh government research report puts the lifespan of units at approximately 25 years⁷. The difference is one of basis, one an average from consumer guidance and the other a figure used in planning research, and a householder should expect somewhere in the 20 to 25 year region, with the service history and the installation quality moving the outcome either way. The lifespan question is covered in detail elsewhere on the site.

What ownership means for energy independence is a mixed picture, and it is worth stating both halves plainly. On the independence side, the household stops buying gas or oil, stops depending on a flue, a gas network or fuel deliveries, and gains a machine that draws its energy from the ambient air, which no one sells and no one can embargo. Government expert guidance frames the technology as part of moving homes off fossil heating, with heat pumps located outside and typically quieter than gas boilers²².

On the dependence side, the household is fully connected to the electricity grid for its heating. A heat pump without electricity produces no heat, so a power cut that a gas boiler would have ridden through becomes a heating outage, unless the household adds storage or backup. The household also depends on the electricity tariff it is on, since the running cost advantage over gas requires a favourable tariff¹¹, and on the manufacturer and installer for parts, servicing and warranty support over a 20 year horizon. Some households pair a heat pump with solar panels and a battery to soften the grid dependence; that reduces, but does not remove, the need for grid electricity in winter.

The balance, then, is this: an air source heat pump converts a household's heating from a fuel that must be burned and bought continuously into an electrically driven system that harvests free heat from the air, at the cost of a permanent dependence on electricity supply and tariff. For a household weighing that trade, the related pages on heat pumps and household energy independence and on replacing a gas, oil or LPG boiler with a heat pump set out the fuller picture.