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Heat pump vs gas boiler: a 1976-1990 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, 1976-1990, 140 m²
- Space-heat demand as found 115 kWh/m²/yr
- SCOP 3.4 @ 45 °C
- Assumed existing boiler efficiency 82 %
- 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 | 115 kWh/m²/yr |
| Annual space heating | 19,320 kWh |
| Hot water | 2,200 kWh |
| Total delivered heat | 21,520 kWh |
| Implied heat loss | 10.7 kW |
| Assumed design flow temperature | 45 °C |
| Assumed SCOP | 3.4 |
| Assumed existing boiler efficiency | 82 % |
How these numbers were produced
- Delivered heat needed: 19,320 kWh (space) + 2,200 kWh (hot water) = 21,520 kWh
- Fuel energy at 82 % seasonal efficiency: 21,520 ÷ 0.82 = 26,244 kWh
- Current annual cost: 26,244 kWh × £0.0733 + £315 standing charges = £2,238
- Heat pump electricity: 21,520 kWh ÷ SCOP 3.4 = 6,329 kWh
- Heat pump annual cost: 6,329 kWh × £0.2611 + £209 standing charge = £1,861
- Running-cost difference: £2,238 − £1,861 = £377
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £377 |
| Simple payback | 14.6 years |
Boiler Upgrade Scheme, £7,500 (air source)
What actually decides it in this house
45 °C puts the SCOP around 3.4 — very close to break-even against gas, and comfortably ahead of oil, LPG or direct electric. The marginal radiator work to get here is usually cheaper than the running-cost difference over ten years.
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