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Heat pump vs gas boiler: a 1919-1944 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, 1919-1944, 140 m²
- Space-heat demand as found 165 kWh/m²/yr
- SCOP 2.8 @ 55 °C
- Assumed existing boiler efficiency 78 %
- 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 | 165 kWh/m²/yr |
| Annual space heating | 27,720 kWh |
| Hot water | 2,200 kWh |
| Total delivered heat | 29,920 kWh |
| Implied heat loss | 15.4 kW |
| Assumed design flow temperature | 55 °C |
| Assumed SCOP | 2.8 |
| Assumed existing boiler efficiency | 78 % |
How these numbers were produced
- Delivered heat needed: 27,720 kWh (space) + 2,200 kWh (hot water) = 29,920 kWh
- Fuel energy at 78 % seasonal efficiency: 29,920 ÷ 0.78 = 38,359 kWh
- Current annual cost: 38,359 kWh × £0.62/l + £209 standing charges = £2,507
- Heat pump electricity: 29,920 kWh ÷ SCOP 2.8 = 10,686 kWh
- Heat pump annual cost: 10,686 kWh × £0.2611 + £209 standing charge = £2,999
- Running-cost difference: £2,507 − £2,999 = -£492
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | -£492 |
| Simple payback | no payback at these assumptions |
Boiler Upgrade Scheme, £7,500 (air source)
What actually decides it in this house
This house is the hard case. At a 55 °C design flow temperature the SCOP lands near 2.8, which is below the 3.6 break-even against gas at the July 2026 cap. A heat pump here costs more to run than a condensing boiler unless you either move to a heat pump electricity tariff or bring the flow temperature down — which means insulation, bigger radiators, or both.
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