Air-to-air or air-to-water?
Until late 2025, this wasn't really a choice — only "wet" heat pumps got the BUS grant. Now both qualify, and the right answer depends on your home.
The short version: air-to-water (the traditional "heat pump") replaces your boiler and runs your existing radiators, suiting most UK homes. Air-to-air (essentially reversible aircon) heats individual rooms with warm-air units and doesn't connect to your plumbing — making it cheaper to install but less complete. The £2,500 air-to-air grant is best suited to flats, smaller homes, and properties where ripping out the radiators would be disproportionate.
What's actually different
Both technologies extract heat from outside air and pump it indoors using a refrigeration cycle in reverse. The difference is what they heat:
- Air-to-water (ASHP): heats water in a tank, which circulates through your existing radiators or underfloor pipes. Provides hot water for showers and taps as well as heating.
- Air-to-air (A2A): blows heated air directly into rooms via wall-mounted indoor units. Doesn't connect to your plumbing. Doesn't heat domestic hot water.
A2A units are also reversible — they cool in summer like an air conditioner. ASHP systems are heat-only (with rare exceptions).
Cost comparison
Indicative 2026 UK installed prices, before grants:
- Air-to-water (ASHP): £10,000–£14,000 for a typical 3-bed semi. Includes new hot water cylinder, controls, possibly some radiator upgrades.
- Air-to-air (A2A): £4,000–£7,000 for a system covering 3–5 main rooms. No hot water, no plumbing changes.
After BUS grants:
- ASHP: £2,500–£6,500 net
- A2A: £1,500–£4,500 net
Important: A2A doesn't provide hot water. You'll still need an immersion heater (cheap to install but expensive to run), a heat pump cylinder (£1,500–£2,500 added), or an alternative like a thermal store. Factor that into the comparison.
Running costs
SCOP (Seasonal Coefficient of Performance) — the ratio of heat output to electricity input — is the key efficiency metric:
- ASHP: SCOP typically 3.0–4.0. Best in homes with low flow temperatures and well-sized radiators.
- A2A: SCOP typically 2.5–3.5. Lower because air-to-air systems run hotter to heat space directly. Also: heat is delivered close to people rather than via a building-wide loop, which can mean less energy is wasted heating empty rooms.
In well-insulated homes, ASHP usually wins on running cost for whole-house heating. In poorly-insulated homes or where you only heat a few rooms regularly, A2A can be competitive or even cheaper.
The case for air-to-water
- Whole-house heating through your existing radiators or underfloor pipes — same experience as a boiler.
- Hot water included via a heat pump cylinder. One system, one bill.
- Higher efficiency if your radiators are correctly sized and your home is reasonably insulated.
- Higher BUS grant at £7,500.
- Better fit for larger homes where heating individual rooms with multiple wall units would be impractical.
The case for air-to-air
- Cheaper install — half the cost or less, even before grants.
- No radiator upgrades needed. Your existing system stays untouched.
- Faster installation — typically 2–3 days versus 1–2 weeks for ASHP.
- Cooling included — runs in reverse during heatwaves, increasingly valuable as UK summers warm.
- Zoned heating — heat only the rooms you're using, when you're using them. Genuinely lower running cost in homes where this matches lifestyle.
- Better for flats where outdoor unit space is constrained and where there's often no space for a hot water cylinder.
- Better for small homes (1–2 bed) where the cost of full ASHP installation is harder to justify.
What about hybrid systems?
You can combine A2A for living spaces with a heat pump cylinder or even keep your existing boiler for hot water. This isn't formally a "hybrid heat pump" in BUS terms (which has a specific definition involving a single integrated control system) but it's a practical setup that some installers offer.
The downside: two systems to maintain, and you don't get the full grant on the boiler retention path.
Decision tree
This is a rough guide — your installer's heat-loss calculation matters more than any general rule.
Choose air-to-water if you
- Live in a 3+ bedroom house with multiple bathrooms
- Have an EPC of C or better (or are willing to upgrade insulation first)
- Currently use radiators or wet underfloor and don't want to change that
- Want one integrated system that handles hot water and heating
- Aren't space-constrained for a hot water cylinder
Choose air-to-air if you
- Live in a flat, small terrace, or 1–2 bed home
- Heat only the rooms you use most (kitchen, living room, bedrooms)
- Are happy to keep an existing immersion or alternative for hot water
- Want summer cooling alongside winter heating
- Need a fast, lower-cost retrofit without disrupting plumbing
- Are in an off-grid property where any heating bill reduction is huge (oil/LPG/electric replacement)
Probably stick with your boiler if you
- Live in an EPC E/F/G property and aren't ready to address insulation first
- Have an oversized radiator system with single panels you can't easily upgrade
- Are moving in <3 years and don't want to disrupt the property
The MCS-installer reality check
Both ASHP and A2A installations need MCS-certified installers and equipment for the grant. The certifications are different:
- ASHP installers need MCS MIS 3005 (heat pumps with hot water).
- A2A installers need MCS MIS 3005-A (air-to-air specific). This certification became mandatory for grant claims when A2A was added to BUS in late 2025, and the installer base is currently smaller than for ASHP.
Practical implication: there are fewer A2A-certified installers on the ground in 2026, especially outside major cities. Lead times can be longer and prices less competitive than the maturer ASHP market.
Our honest take
If your home is suited to ASHP — well-insulated, reasonably-sized radiators, decent space for a cylinder — pick ASHP. The £7,500 grant, hot water inclusion, and higher efficiency tip the balance.
If your home really isn't suited to ASHP — flat with no cylinder space, small property where the install cost is hard to justify, or off-grid heating you want to replace cheaply — A2A is now a serious option in a way it wasn't before the £2,500 grant landed. Use it for what it's good at: zoned space heating, with a separate plan for hot water.
What you should not do is pick A2A purely because it's cheaper, then end up with high running costs because the units are sized wrong, the rooms aren't insulated, or the SCOP is poor. Get heat-loss calculations from any installer offering either system, and reject any quote that hasn't done one.
Run your numbers on our heat pump calculator — pick your system type from the options to see the right grant amount and SCOP applied to your property.
Updated 28 April 2026. Based on current BUS guidance and 2026 installed pricing. SCOP figures are typical; actual performance is installer-dependent.