How to Size a Heat Pump Correctly
Summary
Heat pump size (measured in kW output) should match your home's peak heat loss, not your old boiler size. An MCS-certified installer will calculate this properly. Rule of thumb: 8-10kW for most 3-bed UK homes, but insulation quality matters more than floor area.
Why size matters
An oversized heat pump cycles on and off frequently instead of running continuously. This reduces SCOP (efficiency) and increases wear. An undersized pump cannot keep your home warm on the coldest days. Correct sizing keeps the pump running steadily at its most efficient operating point.
How sizing works
MCS-certified installers use heat loss calculations to work out how much heat your home loses per hour at the UK design temperature (-3°C for most of England, colder in Scotland). This gives the required output in kW. The calculation accounts for:
- Floor area and ceiling height
- Wall, roof, and floor insulation (U-values)
- Window area and glazing type
- Air leakage (draughts)
- Target indoor temperature (usually 21°C living areas, 18°C bedrooms)
Typical sizes for UK homes
| Home type | Typical size (kW) | Notes |
|---|---|---|
| Modern 2-bed flat | 4-6 kW | Good insulation, small heat loss |
| 3-bed semi (1980s+) | 6-8 kW | Decent insulation, cavity walls |
| 3-bed semi (pre-1980) | 8-10 kW | Solid walls, single glazing = higher loss |
| 4-bed detached (older) | 10-14 kW | Large area, exposed walls |
Do not copy your boiler size
Gas boilers are routinely oversized by 50-100% because installers want to avoid callbacks and boilers are cheap to oversize. A 24kW boiler in a 3-bed semi does not mean you need a 24kW heat pump. The actual heat loss is usually 6-10kW. Heat pumps cost more per kW, so oversizing is expensive and harms efficiency.
Get a proper heat loss calculation
Any MCS-certified installer must perform a heat loss calculation to EN 12831 standards before quoting. This is a room-by-room survey, not a guess. If an installer quotes without measuring your home and asking about insulation, walk away. The calculation also determines radiator sizes needed for low flow temperatures.
Sources
- MCS (Microgeneration Certification Scheme), MCS 020 Heat Pump Standard, v2.1
- CIBSE, Domestic Heating Design Guide, 2015
- Energy Saving Trust, Heat pump sizing guidance, 2024
Estimate running costs: Use the calculator to see how SCOP affects annual costs.