As we move towards more sustainable heating systems, air source heat pumps (ASHPs) have gained significant attention.
While air-to-water (A2W) systems are commonly discussed, air-to-air (A2A) heat pumps are less familiar but potentially beneficial for many UK households.
This article explores A2A heat pumps, their pros and cons, and their suitability for different types of homes.
A2A heat pumps operate similarly to air conditioning systems but provide both heating and cooling. The system includes an external unit connected to indoor fans via refrigerant-filled pipes.
A2A heat pumps come in two main configurations:
In addition to heating and cooling, A2A systems offer air filtration and dehumidification, enhancing indoor air quality.
Multi-split A2A heat pump system with separate electric immersion hot water.
Unlike A2W systems, A2A heat pumps usually need a separate hot water source:
Some systems, like the Daikin Multi-plus, integrate a hot water supply, but these are generally suited to small homes or self-contained spaces like loft conversions. For larger homes, a separate hot water supply is required.
Systems like the Daikin Multi+ can connect a hot water cylinder and up to 3 indoor units to a single outdoor unit. However, due to limited capacity they are restricted to smaller homes.
Single-unit systems suit small flats or loft conversions, ideal for heating and cooling open-plan spaces. Individual heaters like infrared panels can supplement heating in bedrooms, with electric towel rails in bathrooms.
For houses, a multi-split system with at least two outdoor units is typically needed. Space constraints and the need to position units near external walls may limit coverage to primary living areas, with additional heaters for secondary rooms like bedrooms.
Indoor units should be positioned to maximise heating and cooling efficiency.
Floor or low wall-mounted units are generally more effective for heating, as they warm cooler air at floor level. Conversely, high wall-mounted units are better for cooling, directing airflow downward.
Proper positioning helps prevent “stratification” – warm air collecting near the ceiling and cool air near the floor – ensuring even temperature distribution.
Indoor units can be wall-mounted at high or low level, or floor-mounted. Note that supplier images always show a concealed pipework installation which is difficult to achieve when retrofitting. Image by Daikin.
A2A systems usually have an individual remote control for each indoor unit.
In multi-split setups, a centralised app-based system can control the entire system. While convenient, some find this less practical than traditional manual controls on other heating systems.
A2A systems may not be ideal for homes with many rooms or limited external wall surfaces. Indoor units need proximity to the outdoor unit for efficiency, which can be challenging in terraced or attached houses.
In some cases, multiple outdoor units may be needed, increasing costs and complexity. A ducted air system may help, but generally, an A2W system is more practical in these situations.
Therefore, A2A systems are best for properties with fewer rooms and detached layouts.
Installing an A2A system can present visual challenges.
Trunked pipe ducts often run along exterior walls and are mounted on interior surfaces. Low-level indoor units minimise trunking visibility by keeping it at skirting level, but higher units require trunking up walls, making it more noticeable.
Additionally, the design of indoor units can clash with period home interiors.
In contrast, A2W heat pumps, connected to radiators or underfloor heating, can be installed more discreetly.
A video showing an A2A multi-split retrofit and the challenges of exterior and interior pipework trunking.
As A2A heat pumps are treated as air conditioning units, planning permission is often required. A2W systems, primarily for heating, qualify under permitted development rights.
READ: Do Air Conditioning Units Need Planning Permission?
Typical A2A system costs:
For comparison, a new combi boiler costs around £2,500 + VAT. VAT does not apply to A2A installation but note these systems are currently ineligible for the Boiler Upgrade Scheme (BUS) grant.
A2A heat pumps are highly efficient, with paired systems reaching a Seasonal Coefficient of Performance (SCOP) up to 6, and multi-split systems typically achieving a SCOP of 3 or 4.
For a typical household using about 11,500kWh per year, running costs are approximately:
This is competitive with gas heating, which averages £816 annually for heating and hot water.
Costs are based on November 2024 energy prices, excluding the cost of a separate hot water system. Efficiency and usage will vary depending on a building’s insulation and heating needs.
A small outdoor A2A unit. Image by Daikin. Note, careful planning is needed to conceal exterior trunking.
Given installation and running costs, payback for an A2A system may take around 16 years for an 8kW unit or over 30 years for a double outdoor system. However, for households replacing inefficient systems or undergoing renovations, ROI can be more favourable – about 8 to 17 years.
Future energy prices, longevity (20–25 years), and environmental benefits can improve ROI, especially as grid electricity becomes greener. A2A systems also offer both heating and cooling, a benefit not reflected in basic payback.
Pairing an A2A heat pump with PV panels can reduce reliance on the grid and lower running costs.
Solar panels can be used to directly support the A2A system during the day, while adding battery storage allows energy to be saved into the evening. Installing smart controls can align the system with peak solar times, maximising efficiencies.
Although initial installation costs may be higher, combining A2A and solar technology offers long-term savings and future-proofs your home with renewable energy.
READ: Do Solar Panels Need Planning Permission? PD & Design Guidance
Finding qualified A2A installers in the UK can be challenging due to limited domestic use. Installers should be F-Gas certified to handle refrigerants safely.
Contact A2A manufacturers for approved installers or consult professional bodies like MCS. Ensure the installer has domestic A2A experience, references, and offers a warranty.
Pros:
Cons:
As we move towards more sustainable heating options, A2A heat pumps offer high efficiency and dual functionality, making them an attractive alternative to traditional gas heating for many homeowners.
They are especially well-suited to properties with fewer rooms, open-plan layouts, or homes without wet central heating.
About the Author
Aron Coates is an architect with over two decades of experience, working on projects ranging from historic building restoration to contemporary home design.
