The UK’s heat pump conversation has, for years, been almost exclusively about air-to-water. Monobloc units, flow temperatures, radiator sizing, MCS accreditation… the vocabulary of A2W has become familiar territory for anyone who follows the renewable heating sector. Air-to-air barely got a mention.
That is changing. Fast.
When the government confirmed that A2A systems would qualify for a £2,500 grant under the Boiler Upgrade Scheme, homeowners began asking questions that the industry was (to put it charitably) not entirely prepared to answer. Our own forums at Renewable Heating Hub filled with threads that exposed genuine confusion: not just among consumers but among professionals who have spent years installing the other kind of heat pump and never touched an air-to-air system in a domestic setting.
So here is the honest, unvarnished picture.
What is an air-to-air heat pump, and how does it work?
An air-to-air heat pump operates on exactly the same thermodynamic principle as an air-to-water system. It extracts heat from outdoor air and moves it indoors, or in summer, reverses the process and cools the home. The difference is in where that heat ends up. An A2W system transfers the heat into water, which then circulates around radiators or underfloor heating. An A2A system delivers the heat directly into the air of the room via one or more indoor units. There is no water circuit involved.
This is not a new technology either. It is not experimental. In Japan and across much of Asia, air-to-air has been the dominant domestic heating and cooling solution since the 1990s. Brands such as Daikin, Mitsubishi Electric and Fujitsu have decades of field data behind them. The UK, as is so often the case with heat pumps, is simply late to formally recognise something that has already been proven at scale elsewhere.
How is an A2A system configured in a home?
The most familiar format is the wall-mounted split unit: a single outdoor compressor connected to one indoor head unit in a room. For whole-home coverage, a multi-split system connects one outdoor unit to several indoor heads across multiple rooms. These can be wall-mounted, floor-standing or ceiling cassettes.
There is also a ducted option… arguably the most elegant solution for a whole-home installation. A single indoor unit is installed in a loft space or ceiling void, and conditioned air is distributed to rooms through concealed ductwork, not unlike a central ventilation system. Done well, this leaves nothing visible in the rooms themselves except a slim grille. It is not cheap, and it requires either an existing void or the willingness to create one, but for new builds or significant refurbishments it is a compelling approach.
For most retrofit situations in existing UK homes, the reality is a combination: wall-mounted units in main living spaces and bedrooms, sometimes with a ducted unit handling upstairs rooms from the loft. The key point is that the system must be properly designed around the specific layout of the house, room by room, not as a rough approximation.
Does it need to be ducted?
No. Individual wall-mounted or floor-standing units can heat rooms without any ductwork whatsoever. Each indoor unit has its own fan, its own temperature control and its own remote or app-based interface. The trade-off is aesthetic: wall-mounted units are visible and, depending on the room, can feel intrusive compared to a radiator tucked under a windowsill.
Some units can be boxed in and painted, though this requires careful attention to airflow, restricting it degrades performance and can cause problems over time. The industry is still catching up on the design integration question, and it is a legitimate concern for homeowners who have invested heavily in an interior.
What about comfort? Is warm air actually comfortable?
This is one of the most common anxieties, and it deserves a direct answer rather than a sales pitch.
Done poorly (a cheap unit running at full blast, set to a high target temperature) A2A heating can feel exactly like you fear it will: a blast of dry, hot air. Done well, with a quality system set to run continuously at a low modulated output rather than cycling on and off, it is genuinely comfortable. The analogy to weather compensation on an A2W system is apt: the principle is the same. Run the system gently and consistently, and the room temperature remains steady. Chase the thermostat aggressively, and you get the kind of temperature swings that make air heating feel unpleasant.
Modern inverter-driven units modulate their output continuously… they do not simply switch on at full power and then cut out. A well-specified system in a well-insulated home, running at a low fan speed and a modest output, is quiet and unobtrusive. The gap in perceived quality between entry-level and premium units is significant. If you are considering A2A for your home, the case for buying at the higher end of the market is strong.
One phenomenon worth understanding is the defrost cycle. In colder, damper conditions, ice can accumulate on the outdoor coil. The system periodically reverses its cycle to melt this, briefly interrupting the flow of warm air indoors. On older or cheaper systems, this could mean a noticeable blast of cool air for a few minutes, roughly once an hour in the worst conditions. Modern systems manage this considerably better, but it is part of the reality of air-source technology in a UK climate and worth being aware of before you commit.
Zoning and control
One of the genuine strengths of a multi-split A2A system is granular control. Each indoor unit can be set individually… different rooms at different temperatures, different schedules. For a household with varied occupancy patterns, this is a real advantage over a conventional wet system with a single zone controller.
The question of whole-home control from a single interface is a reasonable one, and the honest answer is: it depends on the manufacturer and the system specification. Some brands offer sophisticated central controllers or app-based platforms that aggregate all the indoor units into one interface. Others do not, and room-by-room control via individual remotes or wall controllers remains the norm. Third-party integration with platforms such as Tado is possible in some configurations but is not universal and varies significantly by brand and model. This is an area where specifying carefully (and asking the right questions of your installer) matters.
The hot water question
This is the most important practical consideration for the majority of UK homeowners, and it is where A2A’s limitations become most apparent.
A2A systems heat air. They do not produce hot water. If you are replacing a gas combi boiler, you are not just replacing your space heating, you are also replacing your domestic hot water supply. An A2A system addresses only the first part of that equation. Hot water must be handled separately: typically through a heat pump water heater, an exhaust air unit or a smart hot water cylinder with an immersion element, ideally paired with solar.
For the BUS grant to apply, the system must replace fossil-fuel space and water heating entirely. That means the hot water solution is not optional, it is a grant eligibility requirement. For a homeowner with a gas combi and no cylinder, the full cost of transition includes not just the A2A installation but also a separate hot water solution, its installation, any associated plumbing and the disruption that comes with it. The £2,500 headline grant figure needs to be viewed in that wider context.
The BUS grant: what it covers and what it does not
A2A systems qualify for a £2,500 grant under the Boiler Upgrade Scheme in England and Wales, a lower figure than the £7,500 available for air-to-water heat pumps, but a meaningful contribution towards the right type of installation.
The eligibility conditions mirror those for A2W: the installer and the equipment must be MCS certified, the property must have a valid EPC and the system must be replacing a fossil-fuel or direct-electric primary heating source. Crucially, as noted above, the fossil-fuel replacement must cover both space and water heating. A partial solution (A2A for heating with the gas boiler kept for hot water) will not qualify.
The grant is administered through your MCS-certified installer, who handles the application process. The money is deducted from your installation cost upfront rather than claimed back after the fact. You do not see £2,500 arrive in your bank account, it reduces the invoice you pay.
Where does A2A genuinely make sense?
The honest answer is: not everywhere, and not for everyone. But for the right property, the case is strong.
A2A comes into its own in homes that currently rely on direct electric heating (storage heaters, panel heaters or electric underfloor heating) where there has never been a wet hydronic circuit. Installing radiators, pipework, a cylinder and an A2W heat pump in a property that has none of that infrastructure is expensive and disruptive. An A2A system, by contrast, requires only the outdoor unit, the indoor heads and the refrigerant pipework between them. The installation is faster, cleaner and significantly cheaper. The performance improvement over storage heaters is transformational: a modern A2A system can deliver three to four units of heat for every unit of electricity consumed, compared to a direct electric heater which delivers exactly one.
Bungalows and flats with electric heating are a particularly compelling use case. The install cost is often modest (a well-specified single-storey system might cost £5,000 to £8,000 depending on the number of indoor units) and the £2,500 grant covers a meaningful proportion of that. The homeowner gains both cheaper heating in winter and air conditioning in summer, from a single system, with minimal disruption.
For homes with existing wet heating systems (a gas boiler feeding radiators and a hot water cylinder) A2A is more nuanced. If you are undertaking a full refurbishment and rebuilding the interior anyway, incorporating A2A at that point is sensible. If you are simply trying to get off gas with the least disruption, A2W is generally the more straightforward route. The two technologies are not competitors in the same race, they suit different buildings and different circumstances.
The installer supply problem
This is the part of the A2A conversation that tends to be glossed over in the coverage of the BUS grant announcement, and it deserves plain speaking.
There are very few MCS-certified companies currently operating in the domestic A2A space. The commercial air conditioning industry is large, experienced and capable, but the accreditation infrastructure required to access the BUS grant, including MCS, CPS, APHC or BESA membership and the associated compliance overhead, costs between £10,000 and £30,000 per year to maintain per company. For businesses whose engineers bill at commercial rates, the economics of pivoting to domestic A2A installations at lower margins are not straightforward.
This will change. The grant creates a commercial incentive that will draw installers into the domestic market over time. But homeowners enquiring today should be aware that supply is genuinely constrained. If you find an MCS-certified A2A installer, check their domestic experience carefully… there is a significant difference between a company that has spent years commissioning A2A systems in homes and one that has primarily worked in commercial settings and is now moving into domestic for the grant opportunity.
Is A2A the future of UK home heating?
Not universally, and anyone suggesting otherwise is selling something. But it is a genuinely significant technology for a portion of the UK housing stock that has been underserved by the A2W-dominated policy and industry conversation. For the several million homes in the UK that still rely on direct electric heating, A2A is probably the most practical, efficient and cost-effective decarbonisation route available right now.
For the broader housing stock, it is a compelling addition to the installer’s toolkit (particularly as cooling becomes an increasingly serious consideration in UK homes) but it is not a like-for-like boiler replacement in the way the BUS grant framing might imply. The hot water gap is real. The installer supply gap is real. And the premium between an adequate system and a genuinely good one is significant enough to matter.
Go in clear-eyed, specify well and buy quality. The technology is proven. The question is whether the UK industry can build the domestic delivery infrastructure quickly enough to meet the demand that the grant has just created.
You say “Crucially, as noted above, the fossil-fuel replacement must cover both space and water heating”. Could you please point to an authoritative source that makes that clear?
I have direct electric space heating and heat DHW with an immersion heater. I had previously read that I could install an A2A heat pump to replace my space heating and still qualify for the BUS grant even if I kept the immersion heater. So I’d like to be clear in my understanding.
The authoritative reference here is the Ofgem BUS Installers page (https://ofgem.gov.uk/environmental-and-social-schemes/boiler-upgrade-scheme-bus/installers), which sets out the technical requirements that apply to installers when submitting a grant application. It states that the BUS-funded installation must replace all heat-generating components of the original heating system, but with a specific carve-out: “where applicable, you may retain circulation pumps, solar thermal collectors, wood-burning stoves, and supplementary electric heaters, including immersion heaters.”
That carve-out is significant, and it directly addresses your situation. If your primary space heating is direct electric and your hot water is an immersion heater, an A2A system replacing the space heating qualifies… and you are explicitly permitted to retain the immersion heater. The immersion is categorised as a supplementary electric heater, not a competing primary heating system.
For someone (as I understand it) coming off gas, removing the boiler but keeping a gas water heating circuit would very likely fail the “replace all heat-generating components” test.
For someone in your position, already on direct electric throughout, the position is materially different and more straightforward IMO.
I’d strongly recommend your installer confirms this interpretation with Ofgem directly before you commit.
Thanks, that’s a relief. I suspect it’s all hypothetical as it will take too long for any installer to be MCS qualified on the technology for me to be able to get a grant. 🙂
I think, as Simon covered and mentioned in the video, it’ll be more if an installer is bothered to go through the hassle of being MCS certified.