sizingheat lossair source heat pumpUK
What size air source heat pump do I need? A UK sizing guide
Published 10 April 2025
Getting the heat pump size right is the single biggest factor in whether your system runs efficiently and quietly for the next 15 years.
Why sizing matters more for heat pumps than for boilers
A gas boiler can compensate for poor sizing by running at a higher output setting. A heat pump cannot — an oversized unit will cycle on and off frequently rather than running at the low, steady output that gives the best COP.
Sizing by home type (approximate guide)
| Home type | Recommended output |
|---|---|
| 1–2 bed flat, well insulated | 5 kW |
| 2–3 bed semi, average insulation | 7 kW |
| 3–4 bed semi / detached, average insulation | 10 kW |
| 4–5 bed detached, average insulation | 13 kW |
| Large or older property | 16 kW |
The right way to size a heat pump
The only accurate method is a heat loss calculation (BS EN 12831). This takes into account your actual wall, roof, and floor construction, window sizes, and room volumes. Your MCS-certified installer must carry this out before installation.
Use our size guide tool for a quick estimate, then get a proper survey confirmed by your installer.