An air-to-water heat pump extracts heat from outdoor air and transfers it to a water-based heating system — radiators, underfloor circuits and a domestic hot water cylinder. One kWh of electricity typically delivers 3–5 kWh of heat (COP 3.0–5.0). It is Europe's dominant heat pump type for hydronic heating, part of a market that reached 2.88 million units sold in 2025.
How does an air-to-water heat pump work?
An air-to-water heat pump moves heat from outdoor air into heating water through a refrigerant circuit. A fan draws air across the evaporator, where liquid refrigerant boils at low temperature — even at -20 °C, outdoor air still contains usable heat energy. The compressor raises the refrigerant's pressure and temperature, a plate heat exchanger (the condenser) passes that heat to the heating water, and an expansion valve returns the refrigerant to its starting condition.
Because electricity only moves heat rather than generating it, one kWh of input typically yields 3–5 kWh of output — a COP of 3.0–5.0. The "water" half of the name is the defining feature: heat goes into radiators, underfloor circuits and a hot water cylinder, not into blown air.
What flow temperatures can an air-to-water heat pump deliver?
Standard units deliver flow temperatures of 55–60 °C; R290 propane models reach 70–75 °C, enough to replace a boiler on an unmodified radiator system. Efficiency improves roughly 2–3% for every degree the flow temperature drops, so underfloor heating or generously sized radiators remain the efficiency-optimal pairing. For domestic hot water, the unit charges a cylinder to 50–60 °C, with a periodic anti-legionella boost where regulations require one.
In cold climates, EVI compressor models sustain heating operation down to -25 °C. Below about +5 °C in humid weather, the outdoor coil frosts and the unit runs periodic defrost cycles — the reason manufacturers specify a minimum water volume in the system.
How does air-to-water compare with other heat pump types?
| Type | Heat source | Delivers to | Best fit |
|---|---|---|---|
| Air-to-water | Outdoor air | Radiators, underfloor, DHW cylinder | Standard hydronic retrofit and new build |
| Air-to-air | Outdoor air | Indoor fan units | Cooling-led climates, single zones |
| Ground-source (brine-to-water) | Borehole or ground loop | Hydronic system | Highest efficiency, highest install cost |
| Water-to-water | Groundwater, PVT or waste heat | Hydronic system | Where a warm source circuit exists |
Air-to-water wins on installation simplicity: no drilling, no ground loops, one outdoor unit and two water connections. Ground-source achieves higher seasonal efficiency, but the source side costs several times more to install.
Should you choose a monoblock or a split air-to-water unit?
Both constructions deliver identical heat; they differ in where the refrigerant circuit lives. A monoblock heat pump keeps the entire circuit in the outdoor unit and sends heating water through the wall, while a split heat pump pipes refrigerant to an indoor hydrobox. Under the latest EU F-Gas rules, new air-to-water units up to 12 kW placed on the EU market from 1 January 2027 must use refrigerants below GWP 150, which is moving the market — monoblocks first — to R290 propane; installed units are unaffected, and national implementation is still evolving.
How popular are air-to-water heat pumps in Europe?
European heat pump sales grew 13% in 2025 to 2.88 million units across 21 countries (EHPA), with Germany up 50% — the first year heat pumps took half of German new space-heater sales — and France leading in volume at 528,000 units. The 2025 sales cohort alone displaces roughly 2.5 billion cubic metres of imported gas per year. Air-to-water units dominate wherever hydronic heating is the norm. Solimpeks manufactures its air-to-water heat pumps — the Varm Up and the R290-based Varm Boost Series — at its Konya factory, with COP up to 4.92 at A7/W35 (Varm Up Series) and EVI models rated for operation down to -30 °C.
Frequently asked questions
Do air-to-water heat pumps work with existing radiators?
Often yes. A room-by-room heat loss calculation shows which radiators need enlarging; high-temperature R290 and EVI models reach 65–75 °C flow and can run unmodified radiator systems at some efficiency cost. Dropping the design flow temperature to 45–55 °C usually costs a few radiator swaps and repays in running costs.
Do air-to-water heat pumps work below freezing?
Yes. Capacity falls as air temperature drops, but modern units heat effectively at -15 °C and EVI compressor models maintain operation down to -25 °C. Defrost cycles and a defined bivalent or backup strategy are part of normal cold-climate design.
Can one heat pump provide both heating and domestic hot water?
Yes. A motorised diverting valve switches the unit between the heating circuit and a DHW cylinder fitted with an oversized coil designed for heat pump temperatures. The cylinder is charged to 50–60 °C, with periodic anti-legionella boosts.
How long does an air-to-water heat pump last?
15–20 years is a realistic service life for a quality unit. Inverter-driven compressors running long, steady cycles last longest; correct sizing, adequate system volume and routine maintenance protect the lifespan.