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Heat pump vs gas boiler: a 1991-2002 bungalow
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: bungalow, 1991-2002, 90 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 | 90 m² |
| Space-heat demand as found | 95 kWh/m²/yr |
| Annual space heating | 9,832 kWh |
| Hot water | 1,800 kWh |
| Total delivered heat | 11,632 kWh |
| Implied heat loss | 5.5 kW |
| Assumed design flow temperature | 45 °C |
| Assumed SCOP | 3.4 |
| Assumed existing boiler efficiency | 88 % |
How these numbers were produced
- Delivered heat needed: 9,832 kWh (space) + 1,800 kWh (hot water) = 11,632 kWh
- Fuel energy at 88 % seasonal efficiency: 11,632 ÷ 0.88 = 13,218 kWh
- Current annual cost: 13,218 kWh × £0.0733 + £315 standing charges = £1,284
- Heat pump electricity: 11,632 kWh ÷ SCOP 3.4 = 3,421 kWh
- Heat pump annual cost: 3,421 kWh × £0.2611 + £209 standing charge = £1,102
- Running-cost difference: £1,284 − £1,102 = £182
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £182 |
| Simple payback | 30.3 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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