Heat pump in a 1930s semi-detached
The classic British problem house. Cavity walls that may or may not be filled, a suspended timber floor, and radiators sized in the 1970s for a boiler running at 70°C.
With the radiators as they are
Saves £62
a year · existing radiators, average size
With upgraded radiators
Saves £214
a year · SCOP 3.4
What to do first
Cavity wall insulation and loft topping-up first — they cost a fraction of a heat pump and improve the heat pump's numbers when you do fit one. Then size the radiators properly.
Upgrading the radiators is worth about £152 a year in this house — which is the difference between a heat pump being a good idea and a questionable one.
The numbers behind that
- Typical gas use for this kind of house: about 13,500 kWh a year.
- That is roughly 11,475 kWh of actual heat, once boiler losses are taken off.
- A heat pump on the existing radiators would use about 3,957 kWh of electricity to deliver it.
- Break-even electricity price: 27.69p per kWh. The cap is currently 26.11p.
If your break-even price is above the price you actually pay, the heat pump wins. If it is below, it does not — and no amount of enthusiasm changes that arithmetic. A dedicated heat pump tariff can move your real price down enough to flip it.