Exhaust Air Heat Pumps Explained

An exhaust air heat pump is a single indoor appliance that combines ventilation, heating and hot water. It works by recovering heat from the warm, stale air a mechanical ventilation system extracts from kitchens and bathrooms, and using that recovered heat to warm the air in your home and the water in a hot water cylinder¹. Because it harvests heat the house would otherwise throw away, it delivers more heat back into the home than a ventilation system with heat recovery alone, which helps reduce running costs¹.

The trade-off is capacity. The heat an exhaust air heat pump can recover is capped by the amount of air the ventilation system moves, so these units are usually designed for airtight, smaller properties with low heating needs¹. In some cases an exhaust air heat pump can deliver all a home's heating needs, but whether one unit is enough for an entire home is a question an experienced installer needs to answer for the specific property¹. In the UK these systems remain a niche choice compared with the mainstream air source heat pump, and the grant position splits: exhaust air-to-water units qualify for the Boiler Upgrade Scheme grant, while exhaust air-to-air units do not¹.

The way the system operates

An exhaust air heat pump transfers heat from a ventilation system to warm air that heats the home¹. Building regulations in England recognise this heat source directly: approved guidance lists "capturing heat from ambient air, ventilation exhaust air, or a water or ground heat source using a heat pump" among the accepted low-carbon approaches for dwellings⁴. The exhaust air heat pump sits at the ventilation exhaust air end of that list.

Because these units extract heat from ventilated, or exhaust, air leaving a building, they are always combined with a mechanical ventilation system¹. The ventilation side continuously draws warm, stale air out of wet rooms and the kitchen. The heat pump side then boosts the heat that the ventilation unit extracts from that outgoing air, rather than simply dumping it outside as a basic extract fan would¹. The recovered heat is upgraded by the heat pump's refrigeration cycle and delivered back into the home as warm air, and in many systems also into a hot water cylinder¹.

The efficiency of an exhaust air heat pump is affected by three things: the outside temperature, the home's heating needs including room heating and hot water, and how airtight the building is¹. The first two apply to any heat pump; the third is specific to this type, because the heat source is the air leaving the building. A leaky building lets in cold outside air that must be heated from scratch, while an airtight one preserves the warm air the unit is designed to harvest. That is why the design of the home matters as much as the specification of the unit.

One unit providing warmth, hot water and fresh air

Exhaust air heat pumps combine ventilation, heating and hot water in a single unit¹. That is the defining difference from every other heat pump type on this site. An air source heat pump heats a home but does not ventilate it; a mechanical ventilation with heat recovery (MVHR) system ventilates but does not heat water. The exhaust air unit does all three jobs from one box.

On the hot water side, the unit can be used to heat water stored in a hot water cylinder for hot taps, showers and baths¹. On the space heating side, it warms the home through air, which removes the need to install a wet central heating system using radiators or underfloor heating¹. Some types of ducted systems are also known as exhaust air heat pumps and can supply hot water in this way⁵.

The practical consequence is fewer separate systems to buy, install, maintain and eventually replace. For a household, one appliance covers three services that would otherwise need three installations. It also means a single point of failure: if the unit stops, ventilation, heating and hot water all stop together. That concentration of function is the core trade-off of the technology, and it is worth weighing as carefully as the running cost savings.

Suitable and unsuitable homes for exhaust air heat pumps

Exhaust air heat pumps and mechanical ventilation heat recovery systems are usually designed for airtight, smaller properties with low heating needs, and retrofitting them into older houses may not be appropriate¹. That single sentence from Energy Saving Trust sets the boundary for the technology in the UK. The heat available is limited by the ventilation rate, and a large or leaky home simply does not offer enough recoverable warmth in its exhaust air to carry the heating load.

The broader heat pump picture is more forgiving. Heat pumps are technically suitable for most UK homes if installed appropriately, according to a parliamentary research briefing⁶, and air source heat pumps are described as suitable for residential homes, apartments and small commercial buildings in Northern Ireland guidance⁷. Heat pumps are best suited to buildings with good insulation levels, and homes without good levels may need insulation added internally or externally first⁸. An independent assessment of a home's suitability for a heat pump asks whether the home was built since the 1990s, or is older but has had loft and wall insulation installed more recently, and has an EPC rating of A, B or C⁹.

For an exhaust air unit specifically, the bar is higher than for a general heat pump, because the home must be small and airtight, not just insulated. A typical modern flat or small timber-frame house built to current airtightness standards is the natural territory. A pre-1980s house with original single-brick walls and draughty floors is not, unless it has been retrofitted to a standard close to new build. Where a home falls between the two, an installer can advise whether an exhaust air heat pump on its own will be enough to heat the entire home¹.

Purpose built homes compared with later installation

In a new build, the exhaust air heat pump comes into its own. Combining heating and air handling into one unit saves on upfront and installation costs compared with a separate MVHR and heating system¹. The units also remove the need to install a wet central heating system or a gas connection to a new-build property¹, which removes both the cost of a boiler and the standing charge and supply dependence that come with a gas meter.

The design of the new home matters. It is important that the design is as airtight as possible, because the more airtight the building is, the better the MVHR works¹. A ducting system needs to be installed above the ceiling, unless exposed ductwork is acceptable, and metal web floor joist systems make the ducting easier in multi-storey homes¹. The indoor unit itself needs some space where all the ducts flow to and from, ideally close to an exterior wall¹. All of this is far easier to plan into a building at design stage than to thread through a finished house.

In an existing home the picture reverses. Retrofitting ducting above ceilings, achieving the airtightness the system depends on, and finding space for the unit and cylinder in a home never designed for them pushes against the technology's economics. This is why the retrofit market for these units in the UK is small, and why the official deployment statistics, which count retrofit installations in existing properties, capture so little of this technology¹⁰. A household in an older home that wants a heat pump is generally better served by the mainstream options described in the full UK heat pump guide.

Understanding the seasonal performance factor

The designed seasonal performance factor (SPF) of an exhaust air heat pump indicates its running costs¹. The SPF is the ratio of heat delivered to electricity consumed across a whole season. An SPF of 4.0 means that for every 1 kWh of electricity consumed, the heat pump delivers 4 kWh of heat¹¹. The higher the SPF, the less electricity is needed for the same warmth, and the lower the bills.

What counts as a realistic number matters here. Heat pumps tend to run with a seasonal performance factor of 3.1 in practice, according to Energy Saving Trust's answers to common heat pump questions³. However, a good quality heat pump installation that is managed well and controlled appropriately can get an SPF significantly higher than 3.1, sometimes more than 4³. For scheme purposes, older domestic renewable heat incentive rules required a minimum seasonal performance factor of 2.5 for air source heat pumps¹², which sets the floor rather than the target.

For an exhaust air unit, the SPF depends on the same three factors noted above: outside temperature, heating needs, and airtightness¹. An air source heat pump is less efficient in winter when the air temperature is colder¹³, and an exhaust air unit faces the same physics, since its heat source is ultimately the indoor air that outside conditions cool. A household should expect the designed SPF from the system's specification to be the guide to running costs, and should treat any promise of a figure well above 4 with caution unless it reflects a well-controlled installation in a genuinely airtight home. More detail on the measures themselves is on the COP, SCOP and SPF explained page.

Price, funding and the Boiler Upgrade Scheme shortfall

The Boiler Upgrade Scheme provides upfront capital grants towards the cost of installing approved heat pumps and, in limited circumstances, biomass boilers¹⁴. The headline grant is £7,500 off the cost and installation of an air source heat pump¹⁵. For eligible off-gas-grid properties, £9,000 is available towards an air-to-water or ground source heat pump for properties heated by LPG or oil, a rate available until 31 March 2027¹⁶. The scheme also lists £2,500 grants for air-to-air heat pumps in government heat and buildings factsheets¹⁸, though the current scheme rules set the air-to-air position out separately.

For exhaust air heat pumps the position splits by type:

The eligible technologies under the scheme are air-to-water heat pumps, air-to-air heat pumps, ground source heat pumps including water source heat pumps and shared ground loops, and biomass boilers²⁰. Air-to-air heat pumps are only eligible for residential buildings¹⁹. Self-build properties may also qualify, where all other scheme rules are met¹⁹.

The gap is real for households with an exhaust air-to-air unit. The technology that warms air directly, which is how many exhaust air units deliver space heating, sits outside the grant. A household choosing between an exhaust air-to-water unit, which heats a cylinder and qualifies, and an exhaust air-to-air unit, which does not, faces a £7,500 difference in support for what may be a similar installation. The wider grant landscape is covered on the heat pump grants page.

Everyday practicalities: sound, room and the unit

Noise is where an exhaust air heat pump has a clear advantage over its outdoor-unit cousins. Most air source heat pump types need an area outside the home for a unit on a wall or on the ground, with space around it for air flow⁵. The external fan units of air source heat pumps can generate a considerable level of noise¹³, and planning rules respond to that: an air source heat pump cannot exceed 42dB at a distance of 1 metre from the centre of a neighbour's habitable room window, known as the assessment point²¹. In Wales, two units in operation together must not exceed a 40dB limit²², and the Microgeneration Certification Scheme applies a 42 decibel target external noise threshold measured 1 metre from a neighbouring habitable room window²².

An exhaust air heat pump has no outdoor fan unit, so none of these limits bite. The noise that matters is indoors: the unit runs continuously as part of the ventilation system, so the sound of moving air through ducts is part of living with it. Official research on air source heat pump noise found it was a concern for a minority of consumers in the study²³, but that research concerned outdoor units rather than the continuous low-level air movement of a ducted indoor system.

Space is the other practical question. Some indoor space is needed for the unit, where all the ducts flow to and from, ideally close to an exterior wall¹. A hot water cylinder is also part of the system for hot water¹. By contrast, the outdoor compressor unit of a permitted air source heat pump installation is limited in volume: 0.6 cubic metres including housing for a block of flats²⁴, with permitted development guidance giving limits of 0.5 cubic metres on a house and 0.6 cubic metres for a block of flats²⁵. An exhaust air unit trades that outdoor footprint for indoor floor space and ceiling voids full of ducting.

Servicing, fixing and eventually replacing the system

Heat pumps should be serviced according to the manufacturer's advice, usually once a year, by an engineer³. For an exhaust air unit, annual servicing covers both the refrigeration circuit and the ventilation side, since blocked filters or dirty ducts reduce the heat available for recovery. Building regulations guidance in England expects that the person carrying out the work is suitably competent, that heat pump systems have been sized appropriately, and that the system has adequate controls²⁶.

A well-maintained heat pump should last for 20 years or longer². Most systems come with a two to three-year warranty², so the warranty period is short relative to the working life, and the maintenance record matters for the years beyond it. The compressor is the component that defines the economics of repair: replacing it is normally done after about half the service life, about 8 to 12 years²².

Maker guidance on when to replace rather than repair is specific. NIBE's support article states that "it is profitable to repair even if the heat pump is at least 16 years old", and that "after that, you usually replace it if you suffer an expensive repair"²². In other words, a repair on a unit younger than about 16 years can still make economic sense, but an expensive failure beyond that age points to replacement. One fault symptom householders should not ignore is a loss of hot water when winter turns into spring, a known issue on these units²²; it is a matter for a service engineer, not a seasonal quirk to tolerate. The wider fault picture is on the heat pump faults and troubleshooting page.

UK experience and how easy it is to get one

Exhaust air heat pumps are a small presence in a UK heat pump market that is itself still modest. The official deployment statistics count MCS-certified retrofit heat pump installations in existing properties, hydronic heat pumps with a capacity up to 45kW installed in the United Kingdom²⁷. In 2025 Q2, government-supported air source heat pump installations totalled 10,852 across the UK, of which 8,897 were in England, with 104 ground and water source installations in the same quarter²⁸. The previous year's third quarter saw 10,612 air source installations²⁹. The market is growing: a record 125,000 heat pumps were sold in 2025, a 27% increase over 2024, with approximately 45,000 manufactured in the UK¹⁸.

Against that backdrop, exhaust air units barely register in the official counts. The Boiler Upgrade Scheme statistics for May 2022 to July 2026 show zero cumulative air-to-air heat pump vouchers issued in England and Wales¹⁷, which means the air-to-air route, the one most exhaust air units would take for space heating, has drawn no grant support in practice. The units that do appear in the market are typically imported, with NIBE among the makers offering exhaust air heat pumps and support material for them in Europe²².

For a household weighing availability, the practical points are these. Installers with exhaust air experience are far fewer than mainstream air source installers, so the pool of quotes is smaller. Spare parts and service cover depend on the importer's UK presence. And the technology's natural home, the highly airtight small new build, is a segment where developers rather than individual householders usually make the heating choice. None of this makes the technology wrong for a suitable home; it makes the choice harder to exercise and the installer selection more important.

Implications for your home's energy self reliance

An exhaust air heat pump is one of the strongest answers on this site to the question of what a home can do for itself. It removes the need for a gas connection to a new-build property¹, which removes a fuel supply, a meter, a standing charge and exposure to gas price movements in one step. It removes the need for a wet central heating system¹, which removes a boiler and its replacement cycle. And it generates useful heat from air the household has already paid to warm, which is the closest thing to free heat a home gets.

The dependence that remains is electricity. The unit runs on grid power, so a household with an exhaust air heat pump has swapped gas dependence for electricity dependence, and the running cost question becomes the SPF question discussed above. A household pairing the unit with solar panels and a battery would reduce that dependence further, though how far that combination can go depends on the sizing of both, which an installer would need to assess for the specific property¹.

The honest summary is that this technology buys independence cheaply for the right home and buys little for the wrong one. In a small, airtight property, one appliance replaces three, cuts the fuel bill¹ and cuts the gas pipe. In a large or leaky one, the ventilation rate caps the heat available, the unit cannot carry the load, and the household is better served by the mainstream options in the full UK heat pump guide. The technology is not a compromise; it is a specialist, and like all specialists it rewards being matched to the job it was designed for.