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Heat pump in a 1930s semi with no cavity insulation: real numbers
The most common British house in the most common condition. Full calculation: heat demand, flow temperature, SCOP, running cost against gas, and what changes if you fill the cavity first.
A 1930s semi-detached house, 95 m², unfilled cavity walls, double glazing fitted at some point in the 1990s, 250 mm of loft insulation that has settled to nearer 150 mm. This is the modal British house, and it is the case where the answer genuinely is close.
Starting numbers
As found, this house has a space-heating demand around 165 kWh/m² a year, so 15,675 kWh, plus roughly 1,800 kWh of hot water: 17,475 kWh of delivered heat. Implied heat loss is about 8.7 kW at design conditions. With existing radiators and no emitter work, the design flow temperature comes out around 55 °C and the realistic SCOP around 2.8.
| Scenario | SCOP | Electricity | Annual cost | vs gas boiler |
|---|---|---|---|---|
| Gas boiler, 78 % seasonal efficiency | — | — | £1,957 | — |
| Heat pump, existing radiators, 55 °C | 2.8 | 6,241 kWh | £1,838 | £119 |
| Heat pump, three radiators upsized, 50 °C | 3.0 | 5,825 kWh | £1,730 | £227 |
| Cavity filled first, then heat pump at 50 °C | 3.0 | 4,660 kWh | £1,425 | £531 |
Assumptions behind these figures
- 1919–1944 semi-detached, 95 m², 165 kWh/m²/yr space heat as found, 1,800 kWh hot water
- Existing gas boiler seasonal efficiency 78 % — non-condensing, or condensing but poorly commissioned
- Electricity £0.2611/kWh, gas £0.0733/kWh, Ofgem cap 1 July – 30 September 2026
- Cavity fill assumed to cut space-heating demand by 20 %, at the conservative end of published ranges
- Standing charges included on both sides; the gas standing charge is removed in the heat pump rows
- Calculations, not quotes. Your building will differ.
What the table says
Without emitter work the heat pump is close to level with the boiler, and the gas standing charge saving does most of that work. With three radiators upsized it moves clearly ahead. Filling the cavity first changes the picture more than anything else on the list, because it cuts the demand rather than improving the efficiency with which you meet it.
The order of operations that actually pays
- Fill the cavity. Cheapest measure per kWh saved in this house type — and outstanding cavity wall recommendations on your EPC will normally have to be dealt with before a Boiler Upgrade Scheme voucher is issued anyway.
- Top the loft back up to 300 mm. Trivial cost, immediate effect.
- Get a room-by-room heat-loss calculation — after the insulation, not before.
- Upsize the limiting radiators. Usually the north-facing bedroom and the through-lounge.
- Then the heat pump, sized to the post-insulation load — typically 7 kW rather than 9 kW, which is a cheaper unit and a better-behaved one.
Run your own numbers in the running-cost calculator, or see this house type across every era on the house-type pages.
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Questions people ask
Can you put a heat pump in a 1930s semi with unfilled cavity walls?
Yes, but the economics are marginal until the cavity is filled. Filling it first cuts demand by roughly 20 %, lets you fit a smaller heat pump, and is normally required by the EPC condition on the Boiler Upgrade Scheme.