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Heat pump vs gas boiler: a 1967-1975 semi-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: semi-detached house, 1967-1975, 95 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 | 95 m² |
| Space-heat demand as found | 135 kWh/m²/yr |
| Annual space heating | 12,825 kWh |
| Hot water | 1,800 kWh |
| Total delivered heat | 14,625 kWh |
| Implied heat loss | 7.1 kW |
| Assumed design flow temperature | 50 °C |
| Assumed SCOP | 3.0 |
| Assumed existing boiler efficiency | 80 % |
How these numbers were produced
- Delivered heat needed: 12,825 kWh (space) + 1,800 kWh (hot water) = 14,625 kWh
- Fuel energy at 80 % seasonal efficiency: 14,625 ÷ 0.80 = 18,281 kWh
- Current annual cost: 18,281 kWh × £0.0733 + £315 standing charges = £1,655
- Heat pump electricity: 14,625 kWh ÷ SCOP 3.0 = 4,875 kWh
- Heat pump annual cost: 4,875 kWh × £0.2611 + £209 standing charge = £1,482
- Running-cost difference: £1,655 − £1,482 = £173
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £173 |
| Simple payback | 31.8 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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