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Heat pump vs gas boiler: a 1967-1975 detached house
A worked example with this building's own numbers, not an average. Everything below is calculated from the assumptions listed with it, and you can change any of them in the calculator.
Assumptions behind these figures
- House: detached house, 1967-1975, 140 m²
- Space-heat demand as found 135 kWh/m²/yr
- SCOP 3.0 @ 50 °C
- Assumed existing boiler efficiency 80 %
- Ofgem energy price cap, 1 July – 30 September 2026 (direct debit, average). Oil/LPG/coal are editable estimates, not primary-sourced.
- Capital cost £13,000
- Simple payback: no discount rate, no fuel-price inflation, no maintenance difference. Figures are calculations, not quotes.
Fabric and demand
| Treated floor area | 140 m² |
| Space-heat demand as found | 135 kWh/m²/yr |
| Annual space heating | 22,680 kWh |
| Hot water | 2,200 kWh |
| Total delivered heat | 24,880 kWh |
| Implied heat loss | 12.6 kW |
| Assumed design flow temperature | 50 °C |
| Assumed SCOP | 3.0 |
| Assumed existing boiler efficiency | 80 % |
How these numbers were produced
- Delivered heat needed: 22,680 kWh (space) + 2,200 kWh (hot water) = 24,880 kWh
- Fuel energy at 80 % seasonal efficiency: 24,880 ÷ 0.80 = 31,100 kWh
- Current annual cost: 31,100 kWh × £0.0733 + £315 standing charges = £2,594
- Heat pump electricity: 24,880 kWh ÷ SCOP 3.0 = 8,293 kWh
- Heat pump annual cost: 8,293 kWh × £0.2611 + £209 standing charge = £2,374
- Running-cost difference: £2,594 − £2,374 = £220
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £220 |
| Simple payback | 25.0 years |
Boiler Upgrade Scheme, £7,500 (air source)
What actually decides it in this house
At 50 °C the SCOP is about 3.0, still under the 3.6 gas break-even. The gap is small enough that a heat pump tariff, or upsizing the three or four worst radiators, flips the answer. Get the room-by-room calculation before you decide.
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