Air-to-air Heat Pumps
Introduction
During a recent online forum about the UK’s Warm Homes Plan,
a question was asked about the role of air-to-air heat pumps in low-carbon
heating policy. The UK plan will be briefly outlined below, followed by a
discussion of the advantages and disadvantages of air-to-air heat pumps
compared to air-to-water heat pumps, based on an EIRO publication.
The Warm Homes Plan 2026
The plan is discussed in relation to the housing sector in
an article from the Carbon Literacy Project (CLP, 2026). “Homes and buildings
account for roughly one-fifth of UK emissions” and £15 billion is to be
invested with the aim of upgrading five million homes by 2030. The existing Boiler
Upgrade Scheme will continue to offer grants of up to £7,500 per property for
installation of heat pumps and a “£2,500 grant per property will also be
available for air to air (as opposed to air to water) heat pumps which can also
provide cooling.”
The UK Warm Homes Plan (DESNZ, 2026) notes the increase in
sales of heat pumps in the UK in the years 2019 to 2024 and states the aim to deliver
over 450,000 heat pump installations per year by 2030. Reference is made to ground,
water and air source types and the plan claims that “The vast majority of rural
homes are suitable for a heat pump”. Under an existing scheme, 96% of grants
made for heat pump installation were for air source (as opposed to ground or
water source) heat pumps. A typical heat pump system in domestic settings
extracts heat from outside air and transfers it to water which circulates in a
central heating system which heats a building via radiators. The plan confirms
that future grants will include air-to-air heat pumps. These pumps can be “particularly
suitable in smaller properties”, they can be used “for cooling as well as
heating” and are “cheaper to install than a hydronic heat pump” (a heat pump that
uses water to transfer heat into the home).
The UK Office for National Statistics reports that in England
1.4% of dwellings with Energy Performance Certificates use a heat pump. In some
regions the figure is above 9%. For new buildings the figure is over 4% overall
and above 8% in some regions (ONS, 2025).
EIRO Literature review on Air-to-air heat pumps
The Energy Innovation Research Office’s 2025 publication described
the “unique advantages and disadvantages” of air-to-air heat pumps compared to
other types. “Instead of working with radiators (as air-to-water heat pumps
do), they work with air blower units, which can also provide cooling,
dehumidification, air filtration and smart control of individual room
temperatures” (EIRO, 2025).
In its Executive summary the EIRO literature review described
air-to-air heat pumps as uncommon in UK homes though widely used in some
European countries. They are sometimes used in combination with other heating
systems, for example to heat parts of a home that are poorly insulated. Some
types of housing may be particularly suited to air-to-air systems, for example
“flats (particularly those prone to overheating) … and homes without existing
radiator pipework”. Air-to-air heat pumps are widely used in commercial
buildings, so that in the UK more installers are familiar with them than with
air-to-water pumps. EIRO notes that there is limited and contradictory
literature on “the comparative installation costs, operating costs, and energy
efficiency of air-to-air systems for heating compared to air-to-water.” In
Norway, Finland and Sweden, air-to-air heat pumps account for a substantial
proportion of primary heating systems: rising summer temperatures in the UK may
increase demand for the cooling that air-to-air heat pumps can provide, but the
“high proportion of existing wet central heating systems” is a barrier to more air-to-air
heat systems.
A short Introduction leads to a Technical overview,
which distinguishes three main types of system: Mini-split (or single-split) in
which an outdoor unit is connected to a single indoor unit (or cassette) with pipework
transporting the refrigerant between the two; Multi-split, where the outdoor
unit is connected to two or more indoor units; and Variable Refrigerant Flow
(VRF), generally more suited to commercial settings, where the outdoor unit is
connected to many indoor units which can be heated or cooled independently. The
list of major domestic systems providers in the UK includes Mitsubishi,
Samsung, Hitachi, Daikin, and Panasonic. Data on air-to-air multi-split units
by these manufacturers shows a range of heating power from 3.3 to 12 kW, and a
similar cooling power range. The lowest operating air temperature for all the
listed units when they are heating is -15°C; air temperatures below this are not unknown in the UK. All the listed units use R32 refrigerant, classified
as a hydrofluorocarbon (HFC) which puts it in the category of ‘F-gases’; greenhouse
gases with global warming potential (GWP) much higher than that of CO2. Propane
or R290 has a lower GWP and could be an alternative refrigerant.
Energy is wasted
in heating unoccupied rooms, and some systems use motion detectors to identify
when rooms are unoccupied. All the systems studied had links to the user’s
mobile device. Many UK homes, especially those of the elderly, are under-occupied,
and installation of an air-to-air heat pump could provide both energy and cost
savings. Retrofitting can be done when a wet central heating system is still in
situ.
Some areas of
development are described by EIRO: one is the separation of cooling and dehumidifying
operations; a second is technology utilising evaporative cooling that doesn’t
require refrigerants; a third is the solid-state ActiveWall air-to-air heat
pump; a wall integrated system with no pipework, indoor or outdoor unit.
Figures from the International Energy Agency showed very
similar operating costs for air-to-air and air-to-water systems. Manufacturers recommend
yearly servicing of air source heat pumps to ensure efficient operation, as
with annual servicing of gas boilers. Smart controls and connection of units
via Wi-Fi can enable remote fault diagnosis.
Some aspects of greenhouse gas emissions from air-to-air
heat pumps are addressed by EIRO. In the UK, CO2 emissions for heating by an air-to-air
heat pump were almost 3 times lower than from a gas boiler, though the ratio
depends on the way that electricity is generated. No data on embodied carbon
emissions, use in cooling, or refrigerant leakage was available.
The section Integration overview sought to identify the
types of property and existing situations to which air-to-air heat pumps are
best suited. These include many flats; buildings with high heat loss, such as
those with poor insulation due to solid walls and park homes; replacement of existing
electric heaters; and where there is not already a wet central heating system
that would need to be removed.
The scope for installation in England is limited because a
high proportion of dwellings already have central heating with gas fired
boilers and radiators; for these the better option is an air-to-water heat pump
system. Air-to-air heat pumps have an
advantage over air-to-water heat pumps because they can easily provide cooling,
which is more difficult and costly with an air-to-water heat pump and wet
system. However hot water production is not integral to their function, and it accounts
for a significant proportion of the overall demand for domestic heat. Air-to-air
heat pumps can remove dust and particulates, such as pollen and smoke from the
air, benefitting sufferers from asthma and hay fever. Heat pump energy tariffs
are available to users in the UK, but the review found only one dedicated heat
pump tariff compatible with air-to-air pumps.
Market view notes that the UK has one of the greatest
numbers of dwellings among the European countries, at around 29 million, and
has one of the highest proportions of homes in which a natural gas boiler is
the primary heating source. In the years 2018 to 2022 the type of heat pump with
the highest sales in domestic settings in Europe was air-to-air, closely
followed by air-to-water types. The UK sales of air-to-air units are low
compared to Europe generally but are growing steadily, as are sales of air-to-water
types. Germany, France, Italy, and the Netherlands have building counts
comparable to that of the UK and their experiences in deploying and adopting
heat pumps are reviewed in some detail as they are potentially valuable to the
UK. The section concludes with examples of successful case studies in the UK.
Policy and regulation impact predicts that heat will
become a greater problem as climate change brings longer and more intense
heatwaves to the UK. While overheating of buildings is a significant health
risk, UK building policy has so far given only limited attention to cooling in
buildings. Data on cooling practices is limited. The UK takes a ‘passive first’
approach to cooling, managing overheating in new build properties by fixed
shading and glazing designs which do not increase building energy demand. The
effectiveness of passive cooling is limited ultimately by external ambient
temperatures. Existing building regulations allow cooling by air-to-air heat
pumps only in restricted circumstances and after all practical passive cooling
approaches have been considered.
The review’s Conclusions describe air-to-air heat
pumps as having “significant potential in the UK market due to their energy
efficiency, versatility, and ease of installation.” Systems generally provide increased levels of
comfort, heat properties quickly, and their noise levels are acceptable.
Retrofitting can be straightforward but “not challenge-free”. They provide both
heating and cooling capabilities and appear cost competitive with other types
of heat pumps. Questions remain around the phasing out of F-gases and how this
would affect air-to-air heat pumps, and there are gaps between “in-situ and
rated performance”. There are air heat pump systems that can also generate hot
water but availability in the UK is at present limited. “Incentives and
building regulations are less supportive of air-to-air heat pumps than other
types, which may discourage their uptake in situations where they could be
particularly beneficial, and accelerate the decarbonisation of domestic
heating.”
References
CLP, 2026, Warm Homes Plan 2026: What it Means for the
Housing Sector, Carbon Literacy Project, online, accessed 13 March 2026
https://carbonliteracy.com/warm-homes-plan-2026-what-it-means-for-the-housing-sector/
DESNZ, 2026, Warm Homes Plan, UK Government, Department for
Energy Security and Net Zero, online, accessed 13 March 2026
https://www.gov.uk/government/publications/warm-homes-plan
EIRO, 2025, Air-to-Air Heat Pumps: Literature review, Energy
Innovation Research Office, online, accessed 18 March 2026
ONS, 2025, Energy efficiency of housing in England and
Wales: 2025, ONS, October 2025, online, accessed 17 March 2026
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