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Heat pump vs gas boiler: a 2003-2011 terraced 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: terraced house, 2003-2011, 85 m²
- Space-heat demand as found 75 kWh/m²/yr
- SCOP 3.6 @ 40 °C
- Assumed existing boiler efficiency 90 %
- 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 | 85 m² |
| Space-heat demand as found | 75 kWh/m²/yr |
| Annual space heating | 5,419 kWh |
| Hot water | 1,800 kWh |
| Total delivered heat | 7,219 kWh |
| Implied heat loss | 3.0 kW |
| Assumed design flow temperature | 40 °C |
| Assumed SCOP | 3.6 |
| Assumed existing boiler efficiency | 90 % |
How these numbers were produced
- Delivered heat needed: 5,419 kWh (space) + 1,800 kWh (hot water) = 7,219 kWh
- Fuel energy at 90 % seasonal efficiency: 7,219 ÷ 0.90 = 8,021 kWh
- Current annual cost: 8,021 kWh × £0.0733 + £315 standing charges = £903
- Heat pump electricity: 7,219 kWh ÷ SCOP 3.6 = 2,005 kWh
- Heat pump annual cost: 2,005 kWh × £0.2611 + £209 standing charge = £732
- Running-cost difference: £903 − £732 = £170
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £170 |
| Simple payback | 32.3 years |
Boiler Upgrade Scheme, £7,500 (air source)
What actually decides it in this house
At 40 °C the SCOP is about 3.6, which is break-even against gas at the current cap and a clear win against anything else. This is the fabric level where a heat pump stops being an argument.
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