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

An air-to-air heat pump is the technology most people in Britain have already met without knowing its name: it is the wall-mounted unit that blows warm air in winter and cool air in summer, usually sold as "air conditioning". Instead of heating water and sending it to radiators or underfloor pipes, it warms the air inside the building directly, using fans to circulate it¹. It takes warmth from the air outdoors, puts it through a refrigerant, and uses that to warm the air inside the home².

The label matters because it shapes what people think they are buying. These systems are often mislabelled as air conditioning in the UK, and a parliamentary committee was told plainly that "air to air heat pumps, often known as air conditioning units, also provide an alternative, efficient heating option"³. In much of the world the heating function is the point of the machine; in Britain it is treated as a bonus on a cooling product. That has begun to change: air-to-air heat pumps are now an eligible technology under the Boiler Upgrade Scheme, with a £2,500 grant, and MCS certification for them entered its next phase of implementation in September 2026⁴.

Air-to-air heat pumps explained, and why the name misleads

An air-to-air heat pump is a heating system that delivers its heat as blown air rather than as hot water. Where an air-to-water heat pump transfers heat into a wet central heating system, an air-to-air system skips the water entirely: it transports and compresses heat from the air outside, but transfers it to the home as blown air¹⁰. The heat is delivered through pipework, much like air conditioning units, to indoor units that are often wall-mounted⁵.

The confusion around the name is real and has consequences. Because these systems are often mislabelled as air conditioning in the UK, many householders assume they are cooling-only appliances, or that running them for heating is an expensive misuse of a summer gadget¹⁰. Neither is true. The same refrigerant circuit that moves heat out of a room in summer moves it into the room in winter; the machine simply reverses. In countries where summer cooling is routine, the heating side of the same hardware is a standard way to warm homes.

The mislabelling also affects how the technology is treated in schemes and standards. Until recently, air-to-air systems sat awkwardly in UK policy because they looked like air conditioning rather than like a "proper" heating system, even though they are an efficient heating option, as the parliamentary evidence put it³. That is now shifting: MCS, the certification body behind most UK heat pump grants, has been working air-to-air heat pumps into its installer scheme, with the implementation entering its next phase in September 2026⁴. For a householder, the practical point is simpler: if a product is described as an air conditioner with heating, or a split system with a heat pump function, it is very likely an air-to-air heat pump by another name.

Moving warmth from the outdoor air into your living space

The operating principle is the same one behind every air source heat pump. The system absorbs heat from the outside or surrounding air and transfers it into useable heat in the home¹¹. Air source heat pumps operate by absorbing heat from outside air to provide heating¹², and the air-to-air type does this using electricity: it transfers heat from the outside air into the home, and delivers it through pipework to indoor units⁵.

What makes the air-to-air type distinct is the delivery side. An air-to-water system takes the heat drawn from the surrounding air and puts it into water in a wet central heating system¹³, which then flows to radiators or underfloor loops. An air-to-air system instead uses the refrigerant circuit to warm air directly and blow it into the room, so there is no water circuit, no radiators and no cylinder in the heating path¹. That is why the indoor hardware looks like air conditioning: slim wall units with a fan and a filter, not a radiator.

The electricity's role is worth being clear about. The system does not convert electricity into heat the way a fan heater or electric radiator does; it uses electricity to run a compressor and fans that move existing heat from one place to another. Because the air, even cold air, contains heat energy, a small amount of electricity can shift a much larger amount of heat indoors. This is the mechanism behind the efficiency figures in the section below, and it is also why the outdoor unit needs clear space around it: it has to draw a continuous flow of outside air across its heat exchanger¹⁴.

One unit that warms in winter and cools in summer

The feature that most separates an air-to-air system from every other domestic heat pump is that cooling is built in. Air-to-air heat pumps can provide cooling as well, which is very useful in a heatwave⁸. The same indoor units that blow warm air in January blow cool air in July, with no extra hardware to buy, no extra installation and no separate air conditioning system to maintain.

This two-season capability changes the economics of the system. A household that would otherwise consider portable fans or a separate cooling purchase gets both functions from one installation, and an air-to-air heating and cooling system can be especially cost-effective in a smaller house or flat with relatively few rooms, particularly homes that overheat in warm weather⁹. For flats under the roof in summer and cold in winter, one system addresses both problems.

On the control side, heat pumps can use weather compensation, where the system varies its output according to the season rather than running flat out all the time¹⁵. This is called weather compensation, and it is one of the ways a heat pump matches its output to what the house actually needs, rather than cycling hard on and off. In an air-to-air system the indoor units and their thermostats handle room-by-room control, so different rooms can be set differently, which a single radiator circuit cannot easily do.

The seasonal picture is not entirely symmetric. Cooling in a UK summer is a lighter duty than heating in a UK winter, so the system's heating performance and correct sizing matter more than its cooling rating. But the direction of travel in the climate is towards more summer cooling demand, and a system that does both avoids the position of having installed heating that is useless for half the problem.

Three to four times more efficient than a boiler, for space heating alone

Heat pumps are around three to four times more efficient than gas boilers, which should lead to lower household energy bills⁷. The Climate Change Committee, the government's independent advisers, gives that range; Energy Saving Trust states that heat pumps are more than three times more efficient than a gas or oil boiler, helping to lower energy use¹⁴, and the Welsh government's guidance puts it as up to 3 times more efficient than other heating systems¹⁶. Nesta's assessment of air-to-air systems specifically lists them as very energy efficient¹⁰.

What this means in practice is that for every unit of electricity the system consumes, it moves roughly three to four units of heat into the home. A direct electric heater, by contrast, can only deliver one unit of heat per unit of electricity, at most. This is why an air-to-air heat pump heats a room for a fraction of the electricity a fan heater or electric radiator would use for the same warmth, and it is the core of the running-cost case for the technology.

Two qualifications matter. First, these efficiency figures describe space heating. An air-to-air system does not usually provide hot water¹, so the household's water heating is a separate bill, often on direct electricity, which is the most expensive way to heat water. The whole-house comparison with a gas boiler is therefore less favourable than the space-heating comparison alone. Second, efficiency falls when the system is working hardest, in cold weather or in cooling mode, and real performance depends on sizing, insulation and how the units are used. The headline multiple is a fair guide to the physics, not a guarantee of a specific bill.

Best suited to flats, small homes and properties without radiators

Air-to-air heat pumps can provide a good solution for smaller properties, flats, and homes currently without, or poorly suited for, traditional central heating². More often, air-to-air systems are installed in smaller properties, such as flats and park homes, and their outdoor units are typically smaller than those of air-to-water heat pumps¹. Nesta's assessment is that they may be a good option for smaller properties and flats, and that they can be very affordable in small homes, especially studio flats, though costs rise sharply for larger homes¹⁰.

The fit is best where three things line up:

• The home has few rooms to heat, so one or two indoor units cover the space.
• There is no existing radiator system, or one that is poor, so nothing is being thrown away by not using water-based heat.
• The home overheats in warm weather, so the cooling function has real value, not just novelty⁹.

Air source heat pumps more broadly are suitable for residential homes, apartments and small commercial buildings¹⁷, and the air-to-air type is the version of that suited to homes where wet system installation would be disruptive or impossible. For a flat with no gas supply and storage heaters, an air-to-air system replaces direct electric heating with something three to four times more efficient, room by room.

The limits of the fit are equally clear. Costs rise sharply for larger homes¹⁰, because more indoor units and a larger outdoor unit are needed, and a large house with an existing radiator circuit is usually better served by an air-to-water system that uses the radiators it already has. There is also a supply-chain point: indoor heat pump technologies, including air-to-water and air-to-air, currently do not have a very active supply chain in the UK¹⁵, so the range of installers and products is thinner than for the mainstream technologies, though this is changing as MCS certification beds in⁴.

The lack of domestic hot water

The single biggest limitation of an air-to-air heat pump is that it does not usually provide hot water for taps and showers¹. While most models currently available in the UK do not provide hot water², the position is not absolute: some air-to-air systems can be connected to a domestic hot water tank to provide hot water, although not all do this⁹. Nesta's common-questions guidance makes the same point: although some air-to-air heat pumps can also provide hot water, most models available in the UK do not currently do this³.

This gap has to be planned for, not discovered after installation. A household moving from a gas combi boiler to an air-to-air system keeps its space heating but loses its water heating, and the replacement has to be chosen and costed separately. The options include an immersion heater in a cylinder, an electric shower, or a dedicated hot water heat pump or heat pump cylinder. Each of these adds cost and, in the case of a cylinder, adds space requirements that a flat may not have.

The gap also affects the grant arithmetic. The Boiler Upgrade Scheme's £7,500 grant for air-to-water systems reflects a whole-house heating replacement; the £2,500 for air-to-air reflects a space-heating system that leaves water heating to be solved another way⁶. When comparing quotes, a household should compare the total cost of warmth plus hot water across the options, not the heat pump price alone.

Costs involved

The cost picture has two layers: the installation, and the running cost. On installation, Energy Saving Trust gives about £1,900 to heat a single room¹, and Which? reports the same figure, around £1,900 to install a single-unit system to serve a single room⁹. Nesta's common-questions guidance gives a per-room range of £1,700 to £2,700 for systems that do not provide hot water, depending on whether the model is a budget or a premium one³. For whole properties, Nesta's report gives typical installation costs ranging from £2,400 in a one bedroom flat to £8,800 in a four bedroom house⁵, with £8,800 the figure for a large property⁵.

For comparison, air-to-water heat pumps cost around £11,000 to install, with a £7,500 Boiler Upgrade Scheme grant against that⁹. So an air-to-air system is typically the cheaper heat pump route for a small home, though the hot water gap means the comparison is not like for like.

On running costs, a heat pump in an average flat or small home costs £800 to £1,050 a year for space heating on a single-rate tariff, against installation costs of £6,000 to £18,000 across heat pump types⁹. Those figures are for heat pumps generally; an air-to-air system in a small flat sits at the small end of both ranges. VAT is another cost factor: heat pump installations benefit from reduced VAT treatment under the energy saving materials rules¹¹. Where no published price exists for a specific model, prices are installer-quoted, and the figures above are the independent guides' typical ranges, not quotes.

The £2,500 Boiler Upgrade Scheme grant and its conditions

Air-to-air heat pumps are now an eligible technology under the Boiler Upgrade Scheme. The scheme offers £2,500 towards an air-to-air heat pump⁶, and the official notice of approved grant categories confirms the £2,500 value¹⁸. The scheme's eligible technologies include air-to-water heat pumps, air-to-air heat pumps, biomass boilers and ground source heat pumps¹², restated in the July 2026 installer guidance¹³.

The conditions matter as much as the headline figure:

• Residential only. Air-to-air heat pumps are only eligible for residential buildings; this is stated in both the installer guidance and the guidance for property owners¹².
• A specific approved category. The approved grant category for air-to-air is an air-to-air heat pump in an off-gas grid property that is replacing a liquefied petroleum gas heating system¹⁸. This is narrower than the air-to-water route, where the £7,500 grant applies more broadly.
• MCS certification. MCS states that eligible households can receive a £2,500 grant towards the cost of an air-to-air heat pump using the Boiler Upgrade Scheme, once air-to-air heat pumps are available under MCS⁵. The implementation of air-to-air heat pumps into MCS entered its next phase in September 2026, with UKAS approving updates to the MCS Installer scheme to incorporate them⁴.

The Boiler Upgrade Scheme applies in England and Wales. Households in Scotland and Northern Ireland should look to their own schemes; the general position on heat pump grants across the four nations is covered separately. Exhaust air-to-water heat pumps, a related but different technology, are eligible for the £7,500 BUS grant under the scheme's rules¹².

Sound levels, indoor units and air quality

Noise comes from two places in an air-to-air system: the outdoor unit and the indoor units. On the outdoor side, the noise from the outdoor unit of an air-to-air heat pump is likely to be the same as an air-to-water heat pump³. Air source heat pumps' external fan units can generate a considerable level of noise²⁰, which is why planning rules treat noise seriously. Guidance on installations covers existing background noise levels, the characteristics of the noise, noise levels in all operating modes including defrost cycles, the distance between the unit and neighbouring properties, and any noise barriers or mitigation measures²¹.

The specific limits vary by nation and by rule. One adopted standard requires noise levels for an air source heat pump on its own to stay at or below 42 decibels from a metre away from any habitable room²². In Wales, where two heat pumps are installed, the two units in operation together must not exceed the 40 dB limit²³. The MCS 020 noise assessment is the standard installers work to, and the general question of how noisy a heat pump is is covered in detail elsewhere.

The indoor side is where air-to-air differs from other heat pumps: indoor units can be noisier than other heat pumps¹. An air-to-water system's indoor hardware is a silent cylinder and quiet radiators; an air-to-air system's indoor units have fans that run whenever the unit is heating or cooling. In a bedroom, that fan noise is a real consideration, and it is one reason the units are usually sited in living spaces and hallways rather than over beds.

On air quality, the indoor units contain accessible air filters, and most also have deodorising filter panels²⁴. These filters catch dust as air circulates through the room, which is a genuine benefit in a system that moves air continuously, but it only holds if the filters are cleaned, which is covered under maintenance below.

Choosing the right size, position and installation process

Sizing an air-to-air system is a per-room exercise: each indoor unit is matched to the room it serves, and the outdoor unit is matched to the total. One main outdoor unit can be connected to up to five indoor units⁹, which is how a multi-split arrangement covers a whole flat or small house. Getting the sizing right matters twice over: an undersized system will not hold temperature in cold weather, and an oversized one costs more and cycles inefficiently. The general principles of heat pump sizing and heat loss calculations apply here as elsewhere.

On siting, the outdoor unit is usually placed on a wall or the ground outside the property⁵, and it needs some space around it to allow a good flow of air¹⁴. An outdoor unit will usually be roughly one metre high, between 50cm and 100cm wide, and around 50cm deep³, so it is a smaller object than most air-to-water outdoor units, which is part of why these systems suit flats and park homes¹.

Installation itself is comparatively light work. There is no wet system to install or modify: no radiators, no cylinder, no water pipework in the heating path. The work is mounting the outdoor unit, running refrigerant pipework from it to each indoor unit, and mounting the indoor units, which are often wall-mounted⁵. Planning permission is generally not needed for a single air source heat pump on a house under permitted development, but the rules cap what is allowed: 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 dwellinghouse²⁵. Where an application is needed, it should include details of location, design and appearance, noise and vibration, and the efficiency of the unit²⁷.

Once running, the system is fast to respond. In UK trials, rooms typically became comfortable in 10 to 20 minutes, with supply heat arriving in 3 to 4 minutes³. That is a different experience from a wet system warming up from cold, and it suits homes that heat rooms as needed rather than holding a schedule all day.

Yearly servicing and keeping filters clean

The maker's guidance is that a service by a qualified engineer is needed once a year²⁴. During a heat pump service, the technician conducts a comprehensive audit of the system, typically topping up refrigerant, measuring airflow, inspecting filters and parts for dirt, checking wear and tear, and checking remote control operation²⁴. The government's expert Q&A makes the same point at a general level: an annual heat pump service will help it remain clean and ensure it runs efficiently and sounds as quiet as possible²⁸.

Between services, the householder's job is the filters. Most air-to-air heat pumps have accessible air filters and deodorising filters that can be easily removed and cleaned²⁴. The maker's steps are:

1. Turn off the unit.
2. Lift the front panel.
3. Remove the air filters.
4. Remove the deodorising filter panels.
5. Brush or wash the filters.
6. Air dry them away from direct sunlight.
7. Wipe the outside of the unit²⁴.

Regular checks and annual services are what keep the system working as efficiently as it should²⁴. Compared with a gas boiler, there is no combustion to check and no flue, but there is a fan and filter in every occupied room, and a refrigerant circuit that a qualified engineer should handle. The broader question of heat pump servicing and maintenance, and whether servicing affects the warranty, is covered in detail on its own page.

Upcoming refrigerant regulations

The refrigerant inside the system is regulated, and the rules are tightening. From 2025, F gases with a global warming potential above 750 are banned in single split systems that contain less than 3 kg of refrigerant²⁹. That rule already applies to the kind of small split system most homes would get.

Further limits are coming. The government's guidance on F-gas bans sets out the direction of travel for new equipment, and programmes are already preparing for it: the Heat Pump Ready Programme's Stream 2 Wave 2 projects work with low-GWP refrigerants, below 150 GWP³⁰. The practical meaning for a householder is that systems sold in the coming years will move to refrigerants with far lower global warming potential than older air conditioning refrigerants, and the heat pump refrigerants page covers the substances themselves.

The permitted development rules also bear on how many units a property may have: the legislation caps air source heat pumps at one on a non-detached house or block of flats, and two on a detached dwellinghouse²⁵, and the Welsh regulations make the same provision for more than two air source heat pumps not being permitted on a detached dwellinghouse³¹. A household planning a multi-split system with a large outdoor unit should check that the installation stays within these limits.

Its role, and limits, in cutting reliance on the grid

An air-to-air heat pump removes a household's dependence on gas for space heating. It replaces a fossil fuel or direct electric heating system with one that is around three to four times more efficient than a gas boiler⁷, which cuts the electricity needed to hold a home warm and makes the heating compatible with a future in which supply is increasingly renewable. The cooling function adds resilience of a different kind: a home that cannot be cooled in a heatwave is a home that is not self-sustaining in summer either, and this system covers both seasons⁸.

What remains is dependence on electricity. The system runs on mains power, so a power cut stops both heating and cooling, and the running cost is exposed to electricity prices. A household pairing the system with solar panels and a battery, or a time-of-use tariff, can reduce that exposure, but the grid connection itself remains the load-bearing dependency. There is also a dependence on the equipment chain: the refrigerant circuit needs a qualified engineer annually²⁴, the filters need cleaning²⁴, and the UK supply chain for these indoor heat pump technologies is currently not very active¹⁵, so finding products and installers takes more work than for mainstream systems.

The knowledge gap is itself a barrier worth naming. In the government's Winter 2025 attitudes tracker, around a third of owner-occupiers reported not knowing enough about each system to make a decision, 30% for air source and 33% for ground source³². Air-to-air systems, mislabelled as air conditioning and only now entering MCS certification⁴, sit even further outside most householders' picture of heating. The facts on this page are the correction: a technology that heats as well as cools, costs from about £1,900 for a single room¹, carries a £2,500 grant under defined conditions⁶, and leaves hot water as the gap to solve separately².