Air-to-Air Heat Pumps and Heating with Air Conditioning

An air-to-air heat pump takes warmth from the outdoor air, passes it through a refrigerant circuit and uses it to warm the air inside a home. Instead of heating water in radiators or underfloor pipes, it warms the air in the room directly, delivering it through fans from indoor units that are often wall-mounted¹. The hardware is the same family of equipment sold in the UK as air conditioning, which is why these systems are, as Nesta puts it, "often mislabelled as air conditioning in the UK"¹.

Costs are quoted per room rather than per house. Energy Saving Trust puts a single-unit system serving one room at around £1,900². Nesta gives typical installation costs ranging from £2,400 in a one bedroom flat to £8,800 in a four bedroom house⁴, and separately quotes £1,700 to £2,700 per room for systems that do not provide hot water⁵. The Boiler Upgrade Scheme lists £2,500 towards an air-to-air heat pump, against £7,500 for an air source (air-to-water) heat pump⁶.

The defining limitation is hot water. Most models currently available in the UK do not provide it, so taps and showers need a separate system⁸. That single fact shapes where these systems make sense: smaller properties, flats and homes without a conventional wet central heating system.

What an air-to-air heat pump is, and why the label confuses people

Written evidence to Parliament describes air to air heat pumps as "often known as air conditioning units", and treats them as an alternative, efficient heating option¹². The confusion is understandable: the outdoor unit, the refrigerant pipework and the wall-mounted indoor head look exactly like a split air conditioner, because in engineering terms that is what they are. The difference is direction of use. A reversible split system run in heating mode all winter is a heat pump, and the UK market has historically sold it as cooling kit with heating as an afterthought.

Nesta describes the machines as transporting and compressing heat from the air outside and transferring it to a home as blown air¹. MCS describes heat delivered via fans to circulate warm air, rather than through radiators or underfloor pipes, from an outdoor unit usually placed on a wall or the ground outside the property³. Electricity North West puts the same point plainly: "An air-to-air system produces warm air which is circulated by fans to heat your property"¹³.

The contrast is with air-to-water heat pumps, which transfer heat drawn from the surrounding air to water in a wet central heating system and distribute it through the hot water system¹⁴. Both are air source machines; only one of them ends up in a cylinder. The wider family is covered on the air source heat pumps page and on the heat pumps pillar.

How the heat gets from outside air into the room

The physical principle is shared across all air source machines: they absorb heat from the outside or surrounding air and transfer it into useable heat in the home¹⁴. Approved Document L frames it as capturing heat from ambient air, ventilation exhaust air, or a water or ground heat source using a heat pump¹⁴. MCS states the transfer is done using electricity³, which is the point that matters for a household: the machine does not create heat, it moves it, and the electricity buys the movement.

What distinguishes the air-to-air arrangement is the last step. There is no heat exchanger feeding a water circuit, no radiators to size and no cylinder to reheat. Refrigerant runs from the outdoor unit directly to each indoor head, and the head blows warm air into the room. Response is fast: Nesta reports supply heat arriving in 3 to 4 minutes and rooms typically becoming comfortable in 10 to 20 minutes⁵.

Control works differently too. A wet heat pump commonly uses weather compensation, where the system varies the flow temperature according to the season¹⁶. Air-to-air systems are, in Nesta's words, "load compensated" rather than "weather compensated", with each internal head monitoring its room temperature directly¹⁷. In practice that means room-by-room control rather than a single whole-house curve, which suits homes where only some rooms are occupied at a time. More on control strategies is set out under heat pump controls.

Heating in winter, cooling in summer: one machine, two seasons

The same equipment reverses. Which? states plainly that air to air heat pumps "can both heat and cool air depending on what you need"¹⁸, and the Centre for Sustainable Energy notes that air-to-air heat pumps can provide cooling as well, "very useful in a heatwave"¹⁹. Energy Systems Catapult quotes the same duality: "They offer both heating in winter and cooling in summer, making them a smart, future-proof investment"²⁰.

This is the clearest advantage over a wet system, most of which are installed for heating only. A household that would otherwise buy a portable air conditioner in July and run electric heaters in January can do both jobs with one machine, at heat pump efficiency in both directions. The trade-off is that cooling capacity is easy to over-use: summer cooling is electricity demand that the home did not previously have, so lower winter running costs can be partly offset by a new summer load. The heat pump cooling page covers the same question for wet systems.

Efficiency: three to four times a boiler, for space heating only

The Climate Change Committee states that heat pumps are "around three-to-four times more efficient than gas boilers, which should lead to lower household energy bills"¹⁰. Nesta puts the same relationship the other way round: heat pumps use 3 to 4 times less energy to provide the same heat as a boiler, and are three to four times more efficient than other heating systems including other electric ones²¹. Energy Saving Trust says more than three times more efficient than a gas or oil boiler²². The Welsh Government's guide is more conservative, at up to 3 times more efficient¹⁰. Nesta's technology assessment simply records air-to-air heat pumps as "very energy efficient"¹.

The figures differ because they describe different things: laboratory coefficients of performance, seasonal averages across a heating season, and modelled comparisons against different counterfactual fuels. A household should read them as a band, not a number, and expect real performance to depend on the outdoor temperature, how well the home holds heat and how the system is run. The underlying metrics are explained on the heat pump efficiency page.

Two cautions apply specifically to air-to-air. First, the efficiency gain covers space heating only; it does not extend to water heating the machine does not do. Second, most published efficiency evidence for UK homes concerns wet systems. The Heat Pump Association's 2026 report excludes air-to-air heat pumps from its scope entirely²³, which is a fair illustration of how thin the UK evidence base for this technology still is.

Where these systems fit: flats, small homes and homes with no wet system

Nesta identifies smaller properties, flats, and homes currently without, or poorly suited for, traditional central heating⁸. Energy Saving Trust reports that air-to-air systems are more often installed in smaller properties such as flats and park homes². Which? found them particularly well suited to smaller homes and flats, taking up less wall space than traditional radiators²⁴, and especially cost-effective in a smaller house or flat with relatively few rooms, particularly homes that overheat in warm weather¹¹.

Energy Saving Trust's Northern Ireland work makes the structural argument: there are housing types that air-to-water, ground-to-water and water-to-water heat pumps are not suitable for, but which air-to-air can serve, flats among them²⁵. nidirect lists air source heat pumps as suitable for residential homes, apartments and small commercial buildings²⁶. Energy Saving Trust also flags the potential benefits as cooling as well as heating, working in homes with limited space, and suitability for homes without an existing radiator system, while noting there is currently not a very active UK supply chain for indoor heat pump technologies¹⁶.

Cost scales with rooms, not floor area alone. Nesta notes these systems can be very affordable in small homes, especially studio flats, but that costs rise sharply for larger homes¹. Which? also warns that air-to-air heat pumps cannot generally be used to heat larger homes¹⁸.

The hot water gap

Energy Saving Trust states that air-to-air systems "don't usually provide hot water for your taps and shower"². Nesta says most models currently available in the UK do not provide hot water⁸, and its consumer blog adds that although some air-to-air heat pumps can also provide hot water, most UK models do not currently do this⁵. MCS is blunt: "They don't provide hot water."²⁹ Which? notes that some air-to-air systems can be connected to a domestic hot water tank, although not all do this¹¹.

So a household installing air-to-air almost always keeps or adds a second appliance for water. That might be an existing cylinder with an immersion, an instantaneous electric shower, or a dedicated hot water heat pump. The consequence for independence is that a home does not shed its second heat source; it swaps which one does the larger job. It is worth noting that even a wet heat pump does not provide hot water on demand like a combi boiler, so a cylinder is needed either way³⁰; the difference is that an air-to-air machine is not the thing that fills it.

The practical planning point is that indoor heads are generally not suitable for bathrooms or wet rooms, because of high humidity and potential water exposure¹⁷. That constrains layout in exactly the rooms where hot water matters most.

What it costs

For comparison, air-to-water installations are consistently quoted higher: around £11,000¹⁹, an average of £13,000³¹, £8,000 to £12,000³², and £13,600 for a large property, or £6,100 after a £7,500 grant³³. The Centre for Sustainable Energy gives £6,000 to £18,000 to install a heat pump in an average flat or small home, with annual space heating of £800 to £1,050 on a single-rate tariff¹⁹.

The ranges differ because they count different things: some are equipment plus fitting for one room, some are whole-property jobs across several indoor heads, and some include or exclude grant support. Two rules follow. Air-to-air is cheapest where rooms are few, and the advantage narrows as rooms multiply. And because prices are installer-quoted, the figures above are published typical ranges rather than a price any household can rely on. Wider comparison sits on the heat pump cost page.

Grants: the £2,500 Boiler Upgrade Scheme route, and its conditions

Government guidance lists £2,500 towards an air-to-air heat pump under the Boiler Upgrade Scheme⁶, against £7,500 towards an air source heat pump³⁴ and £7,500 for exhaust air-to-water systems⁹. Ofgem's installer guidance names the eligible technologies as "Air-to-Water Heat Pumps, Air-to-Air Heat Pumps, Biomass Boilers and Ground Source Heat-Pumps"⁹, and the July 2026 version repeats air-to-air in the same list³⁵.

Two restrictions matter more than the headline figure.

• Residential only. Ofgem states: "Only residential properties are eligible for air-to-air heat pumps."⁹ The guidance for property owners says the same³⁶.
• A narrow approved category. The notice of approved grant categories describes the qualifying case as "an air-to-air heat pump in an off-gas grid property that is replacing a liquefied petroleum gas heating system"⁷.

Because the Boiler Upgrade Scheme is an England and Wales scheme, households in Scotland and Northern Ireland should check their own national routes; the heat pump grants page sets out what is available in each nation. Background on why air-to-air was excluded for so long is covered under air-to-air heat pump grant eligibility.

Noise, indoor units and air quality

Energy Saving Trust notes that indoor units of air-to-air systems "can be noisier than other heat pumps"², which is the direct consequence of putting a fan in the room rather than a radiator. Outdoors the picture is different: Nesta reports the noise from an air-to-air outdoor unit is likely to be the same as an air-to-water heat pump⁵. Pendle Borough Council warns that external fan units "can generate a considerable level of noise"³⁸.

Planning and permitted development rules turn on measured limits. Bedford Borough Council states that noise levels for an air source heat pump on its own must stay at or below 42 decibels from a metre away from any habitable room³⁹. The Welsh Government's consultation response records that where two units operate together they must not exceed the 40dB limit⁴⁰. Hammersmith and Fulham's guidance expects applications to cover location, design and appearance, noise and vibration, and the efficiency of the unit⁴¹, while Westmorland and Furness lists the assessment factors as background noise, the character of the noise, levels in all operating modes including defrost cycles, distance to neighbours, and any barriers or mitigation⁴². MCS published guidance in November 2025 on mitigating noise from air source heat pumps during installation, service and maintenance, covering both air to water and air-to-air, to support installers applying the Air Source Heat Pump Sound Calculation Standard⁴³. See MCS 020 and heat pump noise for the method.

Sizing, siting and what installation involves

The outdoor unit is usually fixed to a wall or placed on the ground outside the property³, and needs space around it for a good flow of air²². Nesta gives typical outdoor dimensions of roughly one metre high, 50cm to 100cm wide and around 50cm deep⁵, which is smaller than a typical air-to-water external unit at as much as 1 metre by 1.5 metres³⁰. Energy Saving Trust notes that air-to-air outdoor units are typically smaller than those for air-to-water systems².

Indoors, one outdoor unit can serve up to five indoor heads¹¹, so a four-room flat can be covered by a single outdoor machine. Heads are usually wall-mounted³ and, as above, are not generally suited to bathrooms or wet rooms¹⁷.

Permitted development in England limits the number of machines: not more than one air source heat pump on, or within the curtilage of, a dwellinghouse that is not detached or a block of flats, and not more than two on a detached dwellinghouse⁴⁴. Wales made the same two-unit limit for detached dwellinghouses in 2026⁴⁵. Nation-by-nation detail sits on the planning permission pages for England, Scotland, Wales and Northern Ireland.

Energy Saving Trust puts heat pump installation at typically around three days, longer in some cases³⁰. An air-to-air job involves no radiator replacement and no wet pipework, so disruption is concentrated on drilling for refrigerant lines and mounting heads. The general sequence is described under heat pump installation.

Maintenance: an annual service and clean filters

Daikin recommends a service by a qualified engineer once a year, alongside regular owner checks, to keep the system working efficiently⁴⁶. The manufacturer describes an annual service as covering topping up refrigerant, measuring airflow, inspecting filters and parts for dirt, checking wear and tear, and checking that the remote control works⁴⁶. Government guidance makes the general case for all heat pumps: an annual heat pump service "will help it remain clean and ensure it runs efficiently and sounds as quiet as possible"⁴⁷.

Filter cleaning is the owner's routine task, and the manufacturer sets out seven steps: turn off the unit, lift the panel, remove the air filters, remove the deodorising filter panels, brush or wash the filters, air dry them away from direct sunlight, and wipe the outside of the unit⁴⁶. Because an air-to-air system moves room air across those filters continuously, neglecting them reduces airflow and therefore output, in a way that has no equivalent in a radiator circuit. Broader servicing expectations are set out under heat pump servicing and maintenance.

Refrigerant rules ahead

Air-to-air systems carry refrigerant into the building, which places them squarely in the F-gas rules. UK guidance states that from 2025, F gases with a global warming potential above 750 are banned in single split systems that contain less than 3 kg of refrigerant⁴⁸. The direction of travel is towards lower values still: DESNZ's Heat Pump Ready programme defines low-GWP refrigerants as below 150 GWP⁴⁸.

For a household this is mainly a question of what is available to buy and what can be serviced in future. A system installed on a refrigerant heading for restriction may face rising charge costs over its life. The detail of R32, R290 and the regulatory timetable is covered on the heat pump refrigerants page and in R290 vs R32.

What it does, and does not do, for energy independence

An air-to-air heat pump removes gas or oil from the space heating job and replaces it with electricity used three to four times more efficiently than a boiler burns fuel¹⁰. In a flat with no gas connection and no room for a cylinder, that may be the only practical route to a heat pump at all²⁵. Paired with on-site generation, the electricity demand is at least partly addressable by the household, and the fast room-by-room response suits heating only the rooms in use.

The dependencies that remain are real. The machine runs on grid electricity and does nothing without it. Most models leave hot water to a separate appliance and therefore a separate load⁸. The refrigerant circuit is manufacturer-serviced, annually, by a qualified engineer⁴⁶, and the UK supply chain for indoor heat pump technologies is described as not very active¹⁶. And grant support, where it applies at all, is restricted to residential properties and to a narrow off-gas-grid LPG replacement category⁹. The relationship between heat pumps and self-supply is taken further on the heat pumps and energy independence page, and the head-to-head comparison at air-to-air vs air-to-water.