Short answer: a heat pump is worth it in a well-insulated home with the right radiators, and a harder sell in a draughty one with small radiators and no plan to change them. The technology is not the question. The question is what flow temperature your home needs to stay warm — and that comes down to your radiators, not the badge on the heat pump.
Why radiators decide the answer
A gas boiler burns fuel and turns roughly 85–90% of it into useful heat. A heat pump does something cleverer: it moves heat from the outside air, delivering three or four units of heat for every unit of electricity it draws. That ratio is the SCOP (seasonal coefficient of performance), and it is everything. A SCOP of 3.5 means electricity at 26.11p a unit effectively costs 7.5p per unit of heat — comfortably below gas.
The catch is that SCOP collapses when you ask the heat pump for hot water. Small radiators need water at 60–65°C to warm a room; big radiators or underfloor heating do it at 35–45°C. The cooler the water, the more efficient the heat pump. So the same machine might run at a SCOP of 3.9 on underfloor heating and 2.5 on unchanged small radiators — the difference between saving money and losing it.
A worked example: a typical semi
Take a semi-detached house using 12,000 kWh of gas a year through a typical condensing boiler. At the Ofgem cap (7.33p gas, 26.11p electricity), the gas heating costs about £880 a year, plus you would save the gas standing charge of roughly £106 once the meter is gone. Here is what a heat pump does to that bill, depending only on the emitters:
| Emitters | SCOP | Heat pump running cost | Vs gas |
|---|---|---|---|
| Underfloor heating | 3.9 | £683 | saves ~£300 |
| Radiators upgraded for a heat pump | 3.4 | £783 | saves ~£200 |
| Existing radiators, average size | 2.9 | £918 | about level (~£65) |
| Existing radiators, on the small side | 2.5 | £1,065 | costs ~£80 more |
Same house, same warmth, same heat pump. The swing from saving £300 to losing £80 is entirely the radiators. That is why any calculator that gives one headline number without asking about your emitters is not being straight with you. In this example the break-even electricity price for average radiators is around 28p a unit — so at today's cap it just about wins, but a price rise or a cold snap can flip it.
The upfront cost, and the grant
A typical air source heat pump install runs £9,000–£14,000 before any grant, more if radiators and pipework need upgrading. The Boiler Upgrade Scheme takes £7,500 off that in England and Wales — £9,000 if you are off the gas grid (21 July 2026 to 31 March 2027). Crucially, the grant is deducted from the installer's quote before you pay, so you never handle the money and never wait for a refund.
Even so, be honest about payback. If a heat pump saves £250 a year and the net cost after the grant is £4,000, that is a 16-year payback on running cost alone. The strong financial case is rarely "rip out a working boiler to save money". It is: your boiler is dying anyway, you are off the gas grid where oil and LPG are dear, or you are already improving insulation and can size the system properly at the same time.
When a heat pump is genuinely worth it
- Your home is reasonably insulated, or you are improving it as part of the job.
- You can fit larger radiators or underfloor heating, so the system runs at a low flow temperature.
- Your gas boiler is near the end of its life — you are replacing heating anyway.
- You are off the gas grid, where the alternative is expensive oil or LPG.
- You can move to a heat-pump-friendly electricity tariff with cheaper overnight units.
When to wait
If your radiators are small and unchangeable, your home is poorly insulated, your boiler is new, and you are on a standard tariff, the numbers do not fault you for keeping the gas for now. Fix the insulation first — it makes every future heating choice cheaper, heat pump or not. The honest position is that a heat pump is a great decision for many UK homes and a premature one for others, and the only way to know which you are is to run your own figures.