Nesta published a report on 17 February 2026 finding that air-to-air (A2A) heat pumps, often known as air conditioning, can be considered an equally viable decarbonisation option as air-to-water (A2W) systems for almost all UK homes1. The research was carried out with researcher John Ewbank through interviews with homeowners, HVAC engineers and industry professionals, and by reviewing forum discussions, manufacturer datasheets and scientific papers1.
"Our findings suggest A2A heat pumps can be considered an equally viable decarbonisation option as A2W systems for almost all UK homes, and they're particularly advantageous in certain contexts."
The report says A2A installations can be faster and less disruptive than A2W heat pumps, with comparable or lower costs depending on property type, and that they are particularly advantageous where an A2W system would need major pipework upgrades, and in smaller properties or those with lots of open plan space where fewer indoor units are needed1. A2A systems use small refrigerant pipes, typically 6 to 12mm in diameter, between outdoor and indoor units, which the report says can mean minimal visual impact and suit properties where external alterations are restricted1. Multi-split systems use one outdoor unit connected to multiple indoor units, typically two to five, allowing independent temperature control by room or zone1.
The report sets out several limits. Most current A2A systems do not provide hot water, and adding it is an additional cost; many homes with A2A systems continue to burn fossil fuels for hot water, which typically accounts for 20% to 25% of a home's annual gas usage1. A home on mains gas typically uses around 75% of its annual consumption for space heating1. Nesta says displacing that gas with an A2A system may allow homeowners unwilling or unlikely to fully remove gas in the near future to cut direct carbon emissions by approximately 75%, with hot water decarbonised later1. After Boiler Upgrade Scheme (BUS) grants are accounted for, the report says A2A systems with hot water provision are comparable in cost, or more expensive, than most A2W systems with hot water1.
| Point | Nesta's finding |
|---|---|
| Hot water | Most current A2A systems do not provide it; adding it costs extra1 |
| Hot water share of gas use | Typically 20% to 25% of a home's annual gas usage1 |
| Space heating share | Around 75% of annual consumption for a typical mains gas home1 |
| Pipework | Small refrigerant pipes, typically 6 to 12mm diameter1 |
| Planning | A2A systems might be more likely to require planning permission than A2W1 |
Nesta also notes that current planning rules only allow one external heat pump unit per building, or two if the building is detached1. It says A2A systems do not resolve the challenges of installing A2W systems in flats: they still need outdoor space and face the same freeholder consent issues, and external units cannot be mounted on walls at height or hung from windows, so systems are often only practical for flats with roof access, balconies or ground-floor outdoor space1. Indoor units are generally not suitable for bathrooms or wet rooms because of high humidity and potential water exposure1. On skills, the report says the HVAC sector is already close to capacity and the number of engineers will need to grow to meet demand, and that while anyone can take a five-day Category 1 F-Gas course and gain the legal qualification to handle refrigerants, that course does not ensure competency in A2A system design, installation and maintenance1.
Why it matters for households
For a household weighing up how to move away from gas, the report's central claim is that the choice is not only between a full wet-system retrofit and staying on a boiler. An air-to-air heat pump heats rooms directly and can also cool them, and Nesta found homeowners were satisfied with running costs and comfort across different property types1. Because A2A systems are load compensated rather than weather compensated, and work best with a continuous heating pattern but can also be used for zoned and intermittent heating, they may suit households that spend most of the day in one room, are away for long periods, or manage energy use closely1. The trade-off is hot water: keeping a boiler for it means the home still uses fossil fuel, which is why the report frames A2A as an immediate partial cut in direct emissions rather than a complete one1. How these systems compare with wet systems is set out in our guide to air-to-air vs air-to-water heat pumps, and the wider context sits in our heat pumps guide.
What happens next
In November 2025 the UK government announced A2A heat pumps would be eligible for a £2,500 grant under the BUS, available in England and Wales1. Nesta says the planned grant for A2A systems and heat batteries will help mitigate cost disparities between A2A and A2W heat pumps in England and Wales, and that BUS-eligible installations will require Microgeneration Certification Scheme (MCS) certification to maintain installation quality1. The report says it is hard to predict the success of the grant, because HVAC engineers may not want to obtain MCS certification given the added paperwork and more time-intensive heat loss calculations, and because some common A2A system designs may not meet MCS standards, which require an indoor unit in each room1. Nesta adds that the Leasehold and Freehold Reform Act 2024 provides an opportunity to try to reduce heat pump installation challenges associated with freeholder consent1.