Air-to-Water Heat Pumps Explained: How They Work, Costs & Air-to-Water vs Air-to-Air
“Air-to-water” is the heat pump that replaces your boiler — radiators, underfloor heating and hot water, all from the outside air. Here’s how it works, what it really costs after the £7,500 grant, how efficient it is in a British winter, and when an air-to-air system is the smarter choice instead.
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What an air-to-water heat pump is
The name says exactly what it does: it takes heat from the air and puts it into water. That water then does everything your boiler’s water does now — circulates round radiators or underfloor heating, and heats a hot water cylinder for taps and showers. From inside the house the experience is the same wet central-heating system; the difference is that the box outside is quietly extracting heat from the air instead of burning gas.
Because it’s a whole-house heating and hot water solution, air-to-water is the type that UK policy is built around: it’s the “air source heat pump” in the Boiler Upgrade Scheme, in installers’ brochures and in most of our best heat pumps guide. When you hear “heat pump” in a UK context, this is almost always what’s meant.
How it works
Even cold air contains heat — there’s usable energy in it down to about -20°C. The outdoor unit’s fan draws air across a coil of refrigerant, which absorbs that heat and evaporates. A compressor squeezes the gas, raising its temperature sharply. That hot gas passes through a heat exchanger and gives its heat to the water in your heating circuit, then the refrigerant expands, cools and goes round again. It’s the same cycle as your fridge, run in reverse.
The clever part is the maths. The compressor uses electricity, but for every 1kWh of electricity it typically moves 3–4.5kWh of heat into your home. That ratio is the coefficient of performance (COP), and averaged over a whole year it’s the SCOP — the single most important number when you compare systems.
Air-to-water vs air-to-air
This is the comparison most people are actually trying to make, so here it is side by side:
| Air-to-water | Air-to-air | |
|---|---|---|
| What it heats | Water → radiators / UFH + hot water cylinder | Air → blown into rooms via wall units |
| Hot water | Yes — full replacement for a boiler | No — needs a separate solution |
| Cooling | Only some reversible models, via UFH / fan coils | Yes — it’s effectively air conditioning |
| Boiler Upgrade Scheme grant | £7,500 (£9,000 off-gas oil/LPG, to Mar 2027) | £2,500 (since April 2026) |
| Installed cost before grant | £8,000 – £14,000 | £3,000 – £8,000 |
| Net cost after grant | £500 – £6,500 | £500 – £5,500 |
| Efficiency (SCOP) | 3.5 – 4.5 | 3.5 – 4.5 |
| Disruption | Higher — cylinder, pipework, maybe radiators | Low — a unit per room, small pipe through the wall |
| Best for | Whole-house boiler replacement | Flats, small homes, extensions, cooling |
The two are often lumped together but they solve different problems. Air-to-water replaces your boiler outright — one system, heating and hot water, invisible from inside the house. Air-to-air is closer to installing air conditioning that also heats: each room gets a wall unit, you get cooling in summer as a bonus, it’s cheap and quick — but you still need something for hot water (an immersion, a hot-water heat pump cylinder or an existing boiler), and the grant is a third of the size. The efficiency figures are similar because air-to-air doesn’t have to heat water to 45°C, so don’t choose on SCOP; choose on what you need the system to do.
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See My Heat Pump Price After Grant →Cost & the grant
| Air-to-water system | Installed (before grant) | After £7,500 BUS |
|---|---|---|
| 5–8kW (flat, small or modern home) | £8,000 – £10,500 | £500 – £3,000 |
| 8–12kW (typical 3–4 bed) | £10,000 – £13,000 | £2,500 – £5,500 |
| 12–16kW (large or older detached) | £12,000 – £16,000 | £4,500 – £8,500 |
| Add radiator upgrades | + £1,000 – £3,000 | — |
The Boiler Upgrade Scheme pays £7,500 towards an air-to-water (or ground source) heat pump in England and Wales, and since July 2026 an enhanced £9,000 for homes heated by oil or LPG with no mains gas, available until March 2027. You must own the property, be replacing a fossil-fuel or electric heating system, and use an MCS-certified installer; the old EPC requirement was dropped in April 2026. Hybrid systems (heat pump plus boiler) aren’t eligible. The installer claims the grant and knocks it off your invoice — you never handle the money. Air-to-air systems became eligible at £2,500 in April 2026.
Efficiency & running costs
A well-designed air-to-water system achieves a SCOP of 3.5–4.5 — 350–450% efficient against a gas boiler’s ~90%. Electricity costs roughly four times gas per kWh, so at a SCOP of 4 the running cost is about level with gas; at 3.5 slightly more; with a heat-pump tariff (Octopus Cosy, OVO Heat Pump Plus and similar) or solar panels feeding it, noticeably less. Replacing oil or LPG, a heat pump is cheaper to run almost regardless.
Two things decide whether you get a 4.5 or a 3: flow temperature (the cooler the water, the higher the efficiency — more below) and design (correct sizing from a proper heat-loss survey, not a rule of thumb). Brand matters less than people think. For real-world numbers by house type see heat pump running costs.
Radiators, flow temperature & high-temperature units
A gas boiler sends water round at 60–70°C. An air-to-water heat pump is happiest at 35–45°C, and every 5°C lower is worth several percent of efficiency. Cooler water means each radiator gives out less heat, so to keep the room as warm it needs more surface area — which is why installers often replace a few radiators with larger or double-panel ones, and why underfloor heating (which runs at 30–35°C) is the perfect partner. A good heat-loss survey identifies exactly which rooms need it; in a modern house it can be none, in a 1930s semi it’s typically three to five.
If you can’t or won’t change radiators, high-temperature heat pumps (from Daikin, Vaillant, Viessmann, Mitsubishi and others) can run at 65–75°C like a boiler. They work, and for older houses they can be the pragmatic answer — but expect a SCOP nearer 2.5–3 at those temperatures, and higher bills. Lowering the flow temperature is almost always the better investment.
Monobloc vs split
Air-to-water systems come in two layouts. A monobloc has everything in the outdoor unit and sends heated water into the house through insulated pipes — simpler, cheaper, and no refrigerant handling indoors, which is why most UK domestic installs are monobloc. A split system sends refrigerant indoors to a wall-hung unit that does the heat exchange — slightly more efficient over long pipe runs, no risk of the outdoor water pipes freezing, and quieter outside, but it needs an F-gas-qualified installer and costs a little more. For most homes, monobloc; for a long run from the outdoor unit to the plant room, split.
Can an air-to-water heat pump cool?
Sometimes, with caveats. Reversible air-to-water models can chill the water instead of heating it — but standard radiators can’t be used for cooling (they’d drip with condensation), so it only works through underfloor heating (gently) or dedicated fan-coil units. Most UK installers don’t set it up. If summer cooling is genuinely on your list, an air-to-air system is the honest answer: it cools natively because it is air conditioning. Some households run both — air-to-water for heating and hot water, one air-to-air unit in the bedroom for hot nights.
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Air-to-water if…
- You’re replacing a gas, oil or LPG boiler outright
- You want heating and hot water from one system
- You have (or will fit) radiators or underfloor heating
- You want the full £7,500 (or £9,000) grant
- You’re in a house rather than a flat
Air-to-air if…
- You’re in a flat or a small, well-insulated home
- You want cooling in summer as well as heating
- You’re heating an extension, annexe or garden office
- You have no wet system and don’t want one installed
- Budget and speed matter more than hot water
One honest note on noise, since it comes up: a modern air-to-water outdoor unit runs at around 40–50dB a metre away — a fridge hum — and permitted-development rules require it to meet 42dB at the nearest neighbour’s window. See are heat pumps noisy? for the detail.
Brands worth knowing
The UK air-to-water market is dominated by a handful of makers, all of which we’ve reviewed: Mitsubishi Ecodan (the UK’s best-seller), Viessmann and NIBE at the premium end, Panasonic Aquarea and Samsung on value, and Worcester Bosch and Ideal for homeowners who want a familiar boiler brand. If you’re weighing a ground source instead, see Kensa. Our best heat pumps guide ranks them side by side.
The bottom line
If you’re replacing a boiler, an air-to-water heat pump is the one to get. It does the whole job — radiators, underfloor and hot water — at 350–450% efficiency, and with £7,500 (or £9,000) off the price it now costs about the same as a decent boiler for many homes. Get the design right — a proper heat-loss survey, low flow temperature, the right radiators — and it’ll be cheaper to run than gas and far cheaper than oil.
If you’re in a flat, heating a single space, or you want cooling, air-to-air is the smarter, cheaper answer — just budget for hot water separately and expect the smaller £2,500 grant. Either way, the survey is free and it’s the step that turns all the numbers above into yours.
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Air-to-water heat pumps: frequently asked questions
What is an air-to-water heat pump?
An air-to-water heat pump takes heat from the outside air and transfers it into water, which then flows round your radiators or underfloor heating and heats a hot water cylinder — exactly the job a gas boiler does today. It’s what people usually mean by an “air source heat pump” in the UK, and it’s the type that qualifies for the £7,500 Boiler Upgrade Scheme grant.
What’s the difference between air-to-water and air-to-air heat pumps?
Air-to-water heats water for radiators, underfloor heating and your hot water cylinder — a whole-house boiler replacement. Air-to-air blows warm (or cool) air directly into rooms through wall units, like a reversible air conditioner; it can’t make hot water. Air-to-water gets a £7,500 grant, air-to-air £2,500. Air-to-water is the right choice for most homes replacing a boiler; air-to-air suits flats, small well-insulated homes, extensions and anyone who wants cooling.
How much does an air-to-water heat pump cost in the UK?
Typically £8,000–£14,000 installed for a 5–12kW system including the hot water cylinder, before the grant. After the £7,500 Boiler Upgrade Scheme grant that’s £500–£6,500 net, and off-gas-grid homes on oil or LPG can get £9,000 until March 2027. Costs rise if radiators need upgrading or pipework replacing.
Are air-to-water heat pumps efficient in the UK climate?
Yes. A well-designed system achieves a seasonal efficiency (SCOP) of around 3.5–4.5, meaning 3.5–4.5 units of heat for every unit of electricity — even in a British winter, because there’s usable heat in air down to -20°C. The key is running it at a low flow temperature (35–45°C), which is why radiator sizing and good design matter more than the brand.
Can an air-to-water heat pump provide cooling?
Some can. “Reversible” models can circulate chilled water through underfloor heating or fan-coil units in summer, but standard radiators can’t be used for cooling and most UK installs aren’t set up for it. If cooling is a priority, an air-to-air heat pump does it natively — it’s effectively air conditioning that also heats.
Do I need new radiators for an air-to-water heat pump?
Often some, not all. Heat pumps run water cooler than a boiler (around 40–50°C rather than 60–70°C), so radiators need more surface area to give out the same heat. A good installer’s heat-loss survey will identify which rooms need bigger radiators — typically a handful in an older house, sometimes none in a modern one. High-temperature heat pumps can run existing radiators at up to 75°C but at lower efficiency.
Figures are typical 2026 UK estimates for guidance only; costs, efficiency and running costs vary by property, system design, tariff and installer. Boiler Upgrade Scheme amounts and eligibility (England and Wales) are as published by GOV.UK at the time of writing and can change — confirm the current terms before committing. Always use an MCS-certified installer.