The short answer
Both extract heat from the outside air, but they deliver it differently. An air-to-water heat pump heats water for radiators, underfloor heating and a hot water cylinder — it replaces a boiler and runs your wet central heating. An air-to-air heat pump heats (and usually cools) the air directly through indoor fan units, like a reversible air conditioning system, but it does not provide domestic hot water. For a typical UK home wanting whole-house heating and hot water, an air-to-water system is the standard choice, and it is the type eligible for the £7,500 Boiler Upgrade Scheme grant. An air-to-air system suits homes that want fast, zoned heating plus summer cooling and have a separate hot water solution — but it is not eligible for the Boiler Upgrade Scheme. The right choice depends on whether you need hot water and central heating, or air heating and cooling.
Air source heat pumps come in two delivery types, and the difference is whether they heat water or air. That single distinction drives hot water, cooling and grant eligibility. Here is how they compare.
Air-to-air vs air-to-water
- Air-to-water deliversHot water to radiators/UFH + cylinder
- Air-to-air deliversWarm (and cool) air via fan units
- Hot waterAir-to-water yes; air-to-air no
- CoolingAir-to-air yes; air-to-water usually no
- BUS grantAir-to-water yes; air-to-air no
How each one delivers heat
An air-to-water heat pump takes heat from the outside air and transfers it into the water of a wet central heating system. That heated water runs through radiators or underfloor heating and into a hot water cylinder. In effect it does the same job as a gas or oil boiler — whole-house heating plus domestic hot water — but far more efficiently.
An air-to-air heat pump also takes heat from the outside air, but transfers it directly into the indoor air through one or more wall-mounted or ceiling fan units. It is essentially a reversible air conditioning system: in winter it heats the air, in summer it can cool it. Crucially, it heats air only — it does not heat water, so it provides no domestic hot water on its own.
| Factor | Air-to-water | Air-to-air |
|---|---|---|
| Heats | Water (wet central heating) | Air, via indoor fan units |
| Hot water | Yes, via cylinder | No (needs separate solution) |
| Cooling | Usually no | Yes (reversible) |
| Emitters | Radiators / underfloor | Indoor air-handling units |
| Replaces a boiler | Yes | No (heating only) |
| BUS grant | £7,500 | Not eligible |
Indicative comparison for guidance. Sources: Energy Saving Trust; Ofgem.
Hot water, cooling and grants
The hot water question usually decides it. Most UK households want one system that does both space heating and hot water, which is exactly what an air-to-water heat pump provides via its cylinder. An air-to-air system leaves hot water to a separate appliance — an immersion heater, for example — which adds cost and running expense.
Cooling is where air-to-air has the edge: because it is reversible, it can cool rooms in summer, which air-to-water systems generally cannot do through radiators. And grant eligibility is a hard difference — the Boiler Upgrade Scheme funds air-to-water heat pumps (£7,500) but not air-to-air systems, because the scheme is aimed at replacing boilers for whole-house heating and hot water.
Installation, controls and zoning
The two types install very differently inside the home. An air-to-water system connects to the wet central heating circuit and a hot water cylinder, so the indoor work centres on the cylinder, pipework and controls — much like fitting a boiler. Room temperature is managed through thermostats and weather compensation, with the whole house heated from one water circuit.
An air-to-air system uses indoor fan units, one or more per area, each blowing warmed (or cooled) air. That makes it naturally zoned — you can heat or cool individual rooms independently, which can be efficient for spaces used at different times. The trade-off is visible indoor units on walls or ceilings and the absence of any water heating, so hot water must come from a separate appliance.
Maintenance differs too. Air-to-air units have filters that need periodic cleaning, similar to air conditioning, while air-to-water systems share much of the servicing routine of a conventional wet heating system plus the heat pump's own checks. Neither is onerous, but the nature of the upkeep is different.
Which suits which home
For the typical UK home replacing a boiler, an air-to-water heat pump is the standard, grant-eligible solution: it runs the existing wet heating system (with radiators sized appropriately) and provides hot water, all from one unit. It is what most installers fit and what the Boiler Upgrade Scheme supports.
An air-to-air system fits a narrower set of cases: homes that particularly want summer cooling alongside heating, properties without a wet heating system where installing one would be impractical, or spaces like extensions and garden rooms. Because it does not provide hot water and is not grant-eligible, it is rarely the choice for a whole-house boiler replacement. The deciding question is simple — do you need water heating and central heating (air-to-water), or air heating and cooling (air-to-air)?
Frequently asked questions
Does an air-to-air heat pump provide hot water?
No. An air-to-air heat pump heats (and cools) the air through indoor fan units but does not heat water, so it cannot supply domestic hot water on its own — you would need a separate solution such as an immersion heater. If you want one system for both heating and hot water, an air-to-water heat pump is the right type.
Can I get the £7,500 grant for an air-to-air heat pump?
No. The Boiler Upgrade Scheme funds air-to-water (and ground source) heat pumps that provide whole-house heating and hot water, not air-to-air systems. If grant support matters, an air-to-water heat pump is the eligible route, subject to the EPC requirement and an MCS-certified install.
Which is better for a typical UK house?
For most UK homes replacing a boiler, air-to-water is the standard choice — it runs the existing wet heating system and provides hot water, and it qualifies for the £7,500 grant. Air-to-air suits homes that specifically want summer cooling plus heating and have a separate hot water arrangement, but it is not grant-eligible.
Sources & further reading
- Energy Saving Trust — air source heat pumps
- Ofgem — Boiler Upgrade Scheme
- MCS — air source heat pump guidance
Figures on this page are typical UK ranges drawn from published sources and depend on your specific home. They are guidance, not a quotation or guaranteed saving.