What size heat pump do I need?
The right size heat pump is the one that can meet the house's heat loss on the coldest design day, at the flow temperature the radiators were designed for, and no bigger. Undersize it and the house goes cold. Oversize it and it cycles on and off, wears faster and runs less efficiently.
Here is how to size a heat pump properly, and why rules of thumb get it wrong.
Start from the heat loss, not the floor area
The heat pump has to replace the heat the house loses at the outside design temperature. That figure comes from a room-by-room heat loss calculation, not from the old boiler's rating or a watts-per-square-metre guess. Two houses with the same floor area can need very different heat pumps if one has solid walls and single glazing and the other is insulated.
| Method | Why it misleads |
|---|---|
| Matching the old boiler | Boilers are usually far bigger than the heat loss, often 24 kW or more for a house that loses 6 to 8 kW |
| Watts per square metre | Ignores walls, windows, ventilation and the local design temperature |
| Room-by-room heat loss | The method MCS expects, and the only one that also sizes the radiators |
Read the output at your design conditions
A heat pump's nameplate output is measured at test conditions, often 7 °C outside. On a cold day the output falls, and it also depends on the flow temperature. Use the manufacturer's data for the outside design temperature at your site and your design flow temperature, and check the heat pump covers the full design heat loss there.
Example: a house loses 7.2 kW at the design temperature. A model sold as 8 kW may only deliver 6.8 kW at that outside temperature and a 45 °C flow, so it would be undersized. The next size up covers it.
Don't add big margins
- The heat loss calculation already uses a cold design day, so the house only needs full output on a handful of days a year.
- Oversizing makes the heat pump cycle on and off in mild weather, which lowers efficiency and wears the compressor.
- Allow for hot water separately: the cylinder reheat time matters, but it is usually handled by scheduling rather than a bigger unit.
Typical sizes
Most UK homes that have been through a proper heat loss calculation land between about 4 kW and 12 kW. Well-insulated modern homes are often at the bottom of that range, older solid-wall homes at the top. These are typical results, not a substitute for the calculation.
Do it with the free tool
The free tool works out the heat loss room by room from the postcode and your measurements, then carries the total through to heat pump and radiator sizing.
Frequently asked questions
What size heat pump do I need for a 3 bedroom house?
It depends on the heat loss, not the number of bedrooms. Many three-bedroom houses need something between 5 kW and 9 kW, but a draughty solid-wall house can need more and a new build less. A room-by-room heat loss calculation gives the real answer.
Can you size a heat pump from the old boiler?
No. Boilers are almost always much bigger than the house's heat loss, so copying the boiler size would badly oversize the heat pump.
What happens if a heat pump is too big?
It cycles on and off in mild weather, which lowers its efficiency, raises running costs and shortens its life. It also costs more to buy.
What happens if a heat pump is too small?
On the coldest days it can't keep the house at temperature, and it may fall back on an immersion or electric backup heater, which is expensive to run.
Does MCS say how to size a heat pump?
Yes. MCS MIS 3005-D expects a room-by-room heat loss at the design temperature for the site, and the heat pump to meet the design heat loss using the manufacturer's output data at the design conditions.
Related guides
Sources: MCS MIS 3005-D and the MCS Heat Pump Calculator guidance; CIBSE Domestic Heating Design Guide. Free Heat Pump Design follows the MCS method but is not an MCS product and is not endorsed by MCS. Check designs against MCS MIS 3005-D and the official MCS Heat Pump Calculator.