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Heat pump vs gas boiler: a 1991-2002 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, 1991-2002, 140 m²
- Space-heat demand as found 95 kWh/m²/yr
- SCOP 3.4 @ 45 °C
- Assumed existing boiler efficiency 88 %
- 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 | 95 kWh/m²/yr |
| Annual space heating | 15,960 kWh |
| Hot water | 2,200 kWh |
| Total delivered heat | 18,160 kWh |
| Implied heat loss | 8.9 kW |
| Assumed design flow temperature | 45 °C |
| Assumed SCOP | 3.4 |
| Assumed existing boiler efficiency | 88 % |
How these numbers were produced
- Delivered heat needed: 15,960 kWh (space) + 2,200 kWh (hot water) = 18,160 kWh
- Fuel energy at 88 % seasonal efficiency: 18,160 ÷ 0.88 = 20,636 kWh
- Current annual cost: 20,636 kWh × £0.0733 + £315 standing charges = £1,827
- Heat pump electricity: 18,160 kWh ÷ SCOP 3.4 = 5,341 kWh
- Heat pump annual cost: 5,341 kWh × £0.2611 + £209 standing charge = £1,603
- Running-cost difference: £1,827 − £1,603 = £224
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £224 |
| Simple payback | 24.5 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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