What a heat pump actually does
An air-source heat pump doesn’t generate heat like a boiler — it moves it, running refrigeration in reverse to pump warmth from outside air into your radiators and hot water, even at -10°C. The magic number is efficiency: a gas boiler turns 1 unit of gas into ~0.9 units of heat; a well-installed heat pump turns 1 unit of electricity into 3–4 units of heat (a seasonal efficiency, or SCOP, of 3–4). That multiplier is what makes the economics work despite electricity costing more per unit than gas.
The running-cost arithmetic
Under recent price caps, electricity has cost roughly 25–27p/kWh and gas 6–7p/kWh — a ratio of about 4:1. So:
- Heat pump at SCOP 4 → heat costs ~6.5p/kWh: cheaper than gas.
- SCOP 3.5 → ~7.4p/kWh: about level with a good boiler.
- SCOP 2.5 (a poor install) → ~10.5p/kWh: noticeably dearer.
In other words, installation quality is the whole ballgame. Real-world UK monitoring (Ofgem-commissioned and the Electrification of Heat trials) shows well-designed systems routinely achieving 3.5+, and bad ones languishing below 3. Dedicated heat-pump tariffs (cheap electricity in off-peak windows, or heat-pump-specific rates — see the smart tariffs guide) shift the maths further in the pump’s favour — often decisively. On carbon there’s no contest: the UK grid is now clean enough that a heat pump cuts heating emissions by ~70%+ versus gas.
What it costs to install
A typical air-source retrofit runs £10,000–£16,000 before support, including the unit, cylinder and any radiator upgrades. The Boiler Upgrade Scheme (England & Wales) takes £7,500 off upfront via your MCS-certified installer — £9,000 until March 2027 if your home runs on oil or LPG with no mains gas — bringing typical net costs to £3,000–£8,500 — against £2,500–£4,500 for a new gas boiler. Scotland offers grants plus interest-free loans via Home Energy Scotland; the grants guide has every scheme’s details. So the marginal cost over a boiler replacement is now often £1,000–£4,000 — recoverable within the pump’s ~20-year life if the install is good, and instantly if your boiler was due for replacement anyway. Run your own numbers with the same scepticism you’d apply to solar quotes.
Does your house suit one?
Heat pumps deliver water at 35–55°C rather than a boiler’s 60–75°C, so they run “low and slow” — steady background warmth rather than blasts. What matters:
- Insulation first. A leaky house needs high water temperatures, which wrecks the SCOP. Do the insulation basics before, or alongside, the pump.
- Emitter size. Some radiators may need upsizing (typical retrofits change a few, not all); underfloor heating is ideal but not required. Microbore pipework can complicate things.
- Space: an outside unit (fan box, roughly washing-machine-plus sized, quiet-but-not-silent — planning rules now allow within 1 metre of a boundary) and room indoors for a hot-water cylinder. Combi-boiler homes without a cylinder need to find that space; that’s often the real constraint in flats.
- Design quality: insist on a room-by-room heat-loss calculation (not a rule of thumb), an MCS-certified installer, and a quoted SCOP. Ask how the £7,500 is reflected in the price — the grant should reduce your cost, not inflate the quote.
Most UK homes — including most 1930s semis — can be made to work well. The genuinely hard cases are uninsulatable solid-wall homes and small flats with nowhere for a cylinder.
If not now, when?
If your boiler is healthy, waiting is fine — prices are drifting down and installers are getting better. If your boiler is dying, that’s the moment: the grant, the avoided boiler cost and 20 years of falling-carbon electricity all line up. What’s not wise is a distress purchase of whatever’s available on a freezing Friday — get the heat-loss survey done while the boiler still limps.
The one-line summary
A well-installed heat pump now costs about the same as, or less than, gas to run and — with £7,500 off — not vastly more to install than a boiler; the whole outcome hinges on insulation and installer quality, so buy the design survey, not the sales pitch.
Related guides & tools
- UK energy grants and schemes — the Boiler Upgrade Scheme in detail, plus the Scottish and Northern Irish routes
- Insulation and draught-proofing — the prerequisite that makes the SCOP work
- Solar, batteries and smart tariffs — a heat pump raises the value of both
- Smart tariffs and EV charging — the tariffs that tilt heat-pump running costs decisively
- Heating controls — heat pumps run “low and slow”; the settings matter as much as the kit
- EPC ratings — heating systems carry heavy weight in the score
- Solar & Battery ROI Simulator — the same sceptical arithmetic, for the roof
Last reviewed: August 2026.