Air-source heat pump
A heat pump that extracts heat from outdoor air, supplying space heating and hot water (air-to-water) or warm air (air-to-air), and the most widely installed type.
An air-source heat pump (ASHP) uses an outdoor fan coil as its evaporator. Air-to-air units (split or multi-split air conditioners in heating mode) blow warm air indoors; air-to-water units feed radiators, underfloor heating and a hot-water cylinder, directly replacing boilers. Monobloc units keep all refrigerant outdoors; split units pipe refrigerant to an indoor hydraulic module.
Typical SCOP (35 °C, average EU climate): 4.0–5.0
Installed cost (EU/UK, 2024): roughly 8,000–15,000 EUR for a home
Performance drops as outdoor temperature falls, both because the temperature lift grows and because frost forms on the outdoor coil below about 5 °C in humid air, requiring periodic defrost cycles that consume 5–10% of energy. Modern variable-speed (inverter) cold-climate units still deliver COP around 2 at -15 °C and can operate to -25 °C or below; Norway, Sweden and Finland have among the highest heat pump ownership per household in the world.
ASHPs are cheaper and easier to install than ground-source heat pumps but have lower SCOP and need an outdoor unit, which generates 40–60 dB(A) of noise. Good results depend on correct sizing using design heat loss and on low flow temperatures.
Related terms
All terms →- Heat pump
A device that uses a refrigeration cycle to move heat from a cooler source (air, ground, water) to a warmer sink, delivering 2–5 units of heat per unit of electricity.
- Ground-source heat pump
A heat pump that exchanges heat with the ground through buried loops or boreholes, benefiting from stable soil temperatures for high seasonal efficiency.
- COP (Coefficient of Performance)
The ratio of useful heat (or cooling) delivered by a heat pump to the electrical energy it consumes, measured at a specific operating point.
- SCOP (Seasonal COP)
A heat pump's average coefficient of performance over a standardised heating season, weighting test points by how often each outdoor temperature occurs.
- Heating degree days (HDD)
A measure of how much and for how long outdoor temperature falls below a base temperature, used to estimate and normalise building heating demand.