Choosing
9 min readAir-to-air or air-to-water? The honest comparison
Air-to-water replaces your boiler and does hot water. Air-to-air heats and cools individual rooms and doesn't touch your plumbing. Most people aren't choosing between them — here's how to tell which question you're actually asking.
The short answer
Air-to-water replaces your boiler: it heats water for your radiators and your taps, costs roughly £8,000–14,000 installed, and qualifies for the £7,500 grant. Air-to-air heats and cools the air in individual rooms, costs £1,895–5,995 fitted, does nothing for your hot water, and qualifies for £2,500. Both are heat pumps and both work the same way. If you want cooling, only one of them does it. If you want to stop using gas entirely, only the other one does that.
The same machine, plumbed differently
Both are the same physics. A heat pump doesn't make heat, it moves it — pulling warmth out of the outside air using a refrigerant cycle, which is why it can deliver three to five units of heat for every unit of electricity. That part is identical.
Air-to-water — £8,000–14,000
Replaces the boiler. Heats a cylinder, which feeds radiators and taps. Qualifies for £7,500.
Air-to-air — £1,895–5,995
Heats and cools individual rooms. Does nothing for hot water. Qualifies for £2,500.
Same technology, different plumbing — and it is the plumbing that decides which one you want.
The difference is what it does with the heat once it has it. Air-to-water hands it to water, which then goes to radiators, underfloor pipes and a hot water cylinder — so it plugs into the plumbing you already have and replaces the boiler at the end of it. Air-to-air hands it straight to the air in one room through a wall unit, which is why it needs no plumbing, no cylinder and no radiators, and why it can run the cycle backwards in summer and cool instead.
Side by side
| Air-to-water | Air-to-air (what we fit) | |
|---|---|---|
| What it heats | Radiators and hot water | The air in the rooms you put units in |
| Cooling | No | Yes — the same unit, reversed |
| Hot water | Yes, via a cylinder | No. You keep whatever you have |
| Typical installed cost | £8,000–14,000 | £1,895–5,995 |
| BUS grant | £7,500 | £2,500 |
| Install time | A week to a fortnight | One day, two for a multi-split |
| Disruption | Pipework, a cylinder, often new radiators | One wall per room |
| Typical SCOP | 3.0–4.0 | 4.0–5.2 |
| Space needed indoors | A cylinder cupboard | None |
Two rows in that table are worth reading twice. The grant gap is £5,000 in air-to-water's favour, which closes a lot of the price difference if you qualify — and the fossil fuel exclusion means many people don't. And the SCOP figures favour air-to-air, for a reason worth understanding rather than just noting.
Why air-to-air is more efficient, and why that isn’t the point
A heat pump gets less efficient the harder it has to lift the temperature. Air-to-water has to produce water hot enough for radiators — typically 45–55°C in a retrofit, because most British radiators were sized for a 70°C boiler and are too small to work at heat-pump temperatures without being replaced. Air-to-air delivers warm air at around 35–40°C directly into the room, which is a smaller lift, so the same outdoor conditions produce a better coefficient of performance.
- Our units, declared SCOP
- 4.0–5.2Toshiba Seiya through to Mitsubishi MSZ-LN
- Typical air-to-water retrofit
- 3.0–4.0Higher with underfloor heating, lower with small radiators
- What that means per unit of heat
- ~6–7p vs ~8–9pAt 25p/kWh electricity
That is a real advantage and it is also the least important thing on this page. Efficiency at doing something you don’t need is worth nothing: if you want your hot water off gas, an air-to-air unit with a SCOP of 5.2 does not help you, because it will never heat a bath. The running-cost arithmetic is here if you want the numbers behind those figures.
When air-to-water is the right answer
We don't fit it, and these are the situations where you should go and find somebody who does rather than talking to us.
- You want to stop using gas or oil entirely, including for hot water.
- You are heating a whole house rather than particular rooms.
- You qualify for the £7,500 grant — which needs no fossil fuel heating in the property, so in practice it means you are already off gas, or removing it as part of the same job.
- You have somewhere to put a cylinder, and either underfloor heating or radiators big enough to run at a low flow temperature.
- You are doing it anyway as part of a renovation, when the disruption is already priced in.
When air-to-air is the right answer
- Summer is part of the problem. This is the one that decides it for most people.
- One or two rooms are the problem, not the whole house — a loft conversion, a south-facing bedroom, a home office over a garage.
- You are keeping your existing heating for now and want something that helps in the shoulder months without replacing it.
- You want it done in a day, on a date you picked, without pulling up floors.
- You are in a flat or a property where a cylinder and new pipework are not realistic.
The shoulder-month case is the one people underestimate. A single unit heating one room in October and March is doing the job a whole-house system would do at a fraction of the running cost, because you are not warming six rooms to make one of them usable.
Can you have both?
Yes, and for some houses it is genuinely the right answer — air-to-water carrying the heating and hot water, air-to-air handling summer in the rooms that need it. It is two installs and two costs, so it is not a cheap route, but nothing about the two technologies conflicts.
The sequencing matters if you are considering it. Fitting air-to-water first removes the fossil fuel from the property, which is what makes a later air-to-air install grant-eligible at £2,500 — doing it the other way round usually means the air-to-air gets nothing, because the boiler is still there when you apply.
What we'd tell you on the phone
If your first sentence is about being too hot, you want air-to-air and the rest of this page is background. If your first sentence is about your gas bill or your boiler being on its last legs, you want air-to-water and we are the wrong company — we'd rather say so now than after a survey fee.
The one that catches people out is somewhere in between: wanting to use less gas without replacing the boiler. Air-to-air does that well and nobody expects it to, because it is sold as air conditioning — one unit in the room you actually sit in, running through the shoulder months at roughly a quarter the cost of a plug-in heater, and cooling the same room in July. The quote flow prices that in about a minute, and it doesn’t ask for an email address.
Where this comes from
We’d rather you checked. These are the primary sources behind the claims above, in the order they matter.
- Boiler Upgrade Scheme — Ofgem scheme rules£7,500 for air-to-water, £2,500 for air-to-air since April 2026, and the fossil-fuel exclusion that applies to both. Check the current figures before relying on them.
- MCS installation standards for heat pumpsThe certification framework both technologies sit inside, and the reason a grant-eligible install has to be done by a certified installer.
- Manufacturer declared performance — current Daikin, Mitsubishi and Toshiba domestic rangesThe SCOP figures quoted here are from the three units on our shop page and can be checked against it.
- Duneair published pricingOur own installed prices. The air-to-water range quoted is a market observation rather than our figure — we don't fit it.
Not yet independently reviewed. This guide was written from the primary sources listed above and is accurate to the best of our understanding on the date shown. It hasn’t been checked by an independent compliance professional, and it isn’t legal advice. Where a decision turns on it, verify with your local authority or with Ofgem directly — and tell us if we’ve got something wrong.