HeatPumpSwitch › By house type › a pre-1919 semi-detached house
Heat pump vs gas boiler: a pre-1919 semi-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: semi-detached house, pre-1919, 95 m²
- Space-heat demand as found 180 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 | 95 m² |
| Space-heat demand as found | 180 kWh/m²/yr |
| Annual space heating | 17,100 kWh |
| Hot water | 1,800 kWh |
| Total delivered heat | 18,900 kWh |
| Implied heat loss | 9.5 kW |
| Assumed design flow temperature | 55 °C |
| Assumed SCOP | 2.8 |
| Assumed existing boiler efficiency | 78 % |
How these numbers were produced
- Delivered heat needed: 17,100 kWh (space) + 1,800 kWh (hot water) = 18,900 kWh
- Fuel energy at 78 % seasonal efficiency: 18,900 ÷ 0.78 = 24,231 kWh
- Current annual cost: 24,231 kWh × £0.0733 + £315 standing charges = £2,091
- Heat pump electricity: 18,900 kWh ÷ SCOP 2.8 = 6,750 kWh
- Heat pump annual cost: 6,750 kWh × £0.2611 + £209 standing charge = £1,971
- Running-cost difference: £2,091 − £1,971 = £120
Investment and payback
| Installed cost, typical air source | £13,000 |
| Grant | −£7,500 |
| Net capital cost | £5,500 |
| Annual running-cost saving | £120 |
| Simple payback | 46.0 years |
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.
Advertising
Get quotes for your property
Compare at least three installers. Ask each one for the heat-loss calculation and the design flow temperature in writing — that number decides your running cost more than the brand on the box.
Ad. If you request a quote through this link we may be paid a commission by the installer. It does not change your price, and we do not rank companies by how much they pay.