An air-to-air heat pump blows warm air through indoor fan units and cannot make domestic hot water; an air-to-water heat pump heats water for radiators, underfloor circuits and a cylinder. Air-to-air is cheaper, cools well and posts higher datasheet efficiency (SCOP 4.0–5.2). Air-to-water covers heating and hot water in one system and is the type almost every national grant scheme funds.
What is the difference between air-to-air and air-to-water?
The output medium, and everything that follows from it. An air-to-air heat pump delivers heat as warm air through indoor fan units — the same hardware as a split air conditioner, run in reverse. An air-to-water heat pump delivers heat as hot water into radiators, underfloor circuits and a hot water cylinder.
Air-to-air has no water circuit, so there is no cylinder, no buffer tank, no glycol, no circulation pump and no wet commissioning — and also no domestic hot water. Air-to-water plumbs into the heating system the building already owns, which is why it dominates the European retrofit market.
Which is more efficient?
On the datasheet, air-to-air wins. On the annual bill, air-to-water usually does, because the comparison is not like for like.
| Air-to-air | Air-to-water | |
|---|---|---|
| Declared heating SCOP | 4.0 – 5.2 | 3.2 – 4.5 |
| Domestic hot water | Not possible | COP 2.5 – 3.5 |
| Heat distribution | Fan cassettes, one per room or zone | Existing radiators or underfloor |
| Summer cooling | Yes, with dehumidification | Yes with reversible units, via fan coils or underfloor (dew-point limited) |
| Indoor equipment | A cassette in every heated room | Cylinder plus a compact hydraulic unit |
| Installation cost | Lower | Higher |
| Grant eligibility | Often excluded or lower rate | Usually eligible |
Air-to-air posts SCOP 4.0–5.2 because it delivers 35–40 °C air rather than 45 °C water — a smaller temperature lift. But it only heats the rooms you fit cassettes in. Hallways, bathrooms and closed bedrooms stay cold, and hot water still comes from an immersion element working at an effective COP of 1.0. Domestic hot water is typically 15–25% of a European home's heat demand; covering that share at COP 1 instead of COP 3 erases most of the headline advantage.
What about hot water?
An air-to-air system cannot make any. It therefore needs a second appliance — an immersion cylinder, the gas boiler you were trying to remove, or a separate hot water heat pump — and that third box is usually where the cost advantage disappears. It also splits responsibility for legionella control across two systems; see what temperature kills legionella in a heat pump system?.
An air-to-water unit charges the cylinder through a large-surface coil heat exchanger at 48–52 °C, at COP 2.5–3.5, and can share that cylinder with solar thermal or PVT collectors. Solimpeks builds heat pump cylinders and hygienic combination tanks around exactly that duty, with coil surfaces sized for low-temperature charging rather than boiler-era 80 °C water.
Which one do grants pay for?
Almost all of them are written around air-to-water. France's MaPrimeRénov' funds air-to-water heat pumps and excludes air-to-air units, the UK Boiler Upgrade Scheme has paid a fixed £2,500 for air-to-air units in homes since 28 April 2026 against £7,500 for air-to-water, and Germany's BEG scheme is built around systems supplying space heating and hot water. Because grants are commonly worth several thousand euros, eligibility often decides the comparison on its own — check the current official rules for your country, and see UK & Ireland heat pump grants 2026 and MaPrimeRénov' 2026.
Which should you choose?
Choose air-to-air when the building has no wet heating system at all — an apartment on electric panel heaters, a holiday home, an office or a single room; when summer cooling is a genuine requirement rather than a bonus; or when the budget cannot stretch to a cylinder and pipework.
Choose air-to-water when a wet system already exists, when hot water has to be covered by the same appliance, when you want the grant, or when you intend to add solar thermal, PVT or a larger store later. Only a water-based system can be combined with collectors, because only water carries heat between them.
Can you use both?
Yes, and in southern and Mediterranean climates it is common: an air-to-water system for heating and hot water, plus one or two air-to-air splits in the rooms that need cooling in July. That combination costs more than either alone but avoids the compromise of underfloor cooling, which is limited by dew point, and keeps hot water on the efficient machine all year.
Frequently asked questions
Can an air-to-air heat pump heat water?
No. It transfers heat directly into room air and has no water circuit, so domestic hot water needs a separate appliance — an immersion cylinder, a boiler, or a dedicated hot water heat pump. That second appliance is the main hidden cost of the air-to-air route.
Is air-to-air cheaper than air-to-water?
To install, yes — usually substantially, because there is no cylinder, no pipework and no wet commissioning. Over a full year the gap narrows sharply once hot water is heated by an immersion element and once grant eligibility is taken into account.
Is air-to-air more efficient than air-to-water?
For space heating alone, yes: declared SCOP is typically 4.0–5.2 against 3.2–4.5, because warm air needs a smaller temperature lift than hot water. Counting hot water and unheated rooms, a well-designed air-to-water system normally uses less electricity overall.
Can I get a grant for an air-to-air heat pump?
Sometimes, at a lower rate. France's MaPrimeRénov' excludes air-to-air units, the UK Boiler Upgrade Scheme has offered a fixed £2,500 for air-to-air units in homes since 28 April 2026 (against £7,500 for air-to-water), and most other national schemes require the system to supply hot water as well as heating. Always check the current official scheme rules before ordering.