Replacing a tired boiler is one of the biggest spending decisions a homeowner makes, and the choice is no longer just between two gas boilers. Air source heat pumps are now routine installs in England and Wales, helped by a government grant, but they behave very differently from a boiler and they do not suit every house equally. This article sets out how the two compare on running cost, upfront cost, comfort, space and disruption, and where the decision genuinely hinges on your home rather than on the headlines.
How each system works, in one paragraph each
A gas boiler burns gas to heat water to 60–80°C, which then circulates through radiators. A modern condensing boiler converts roughly 85–92% of the energy in the gas into heat in real conditions, so each kilowatt-hour of gas gives somewhat less than a kilowatt-hour of heat.
An air source heat pump (ASHP) uses electricity to move heat from outdoor air into the house, much like a fridge in reverse. For each kilowatt-hour of electricity it uses, it delivers several kilowatt-hours of heat. That ratio, measured over a season, is the seasonal coefficient of performance (SCOP or SPF). In well-designed homes a figure of around 3 to 4 is typical; poorly designed systems with high flow temperatures do worse. The lower the water temperature you ask the heat pump to produce, the better the ratio, which is why house design matters so much.
Running costs: the arithmetic that decides it
The comparison comes down to the price of a kilowatt-hour of electricity relative to gas, and to the efficiency of each system. Use the formula:
Cost per kWh of heat = unit price ÷ efficiency
The following uses round, made-up unit prices purely to show the method. Look up your own tariff before making any decision, since the Ofgem price cap changes every quarter.
- Assumed gas price: 6.0p per kWh. Boiler efficiency 88% gives 6.0 ÷ 0.88 = 6.8p per kWh of heat.
- Assumed electricity price: 24p per kWh. Heat pump SCOP 3.0 gives 24 ÷ 3.0 = 8.0p per kWh of heat. At SCOP 3.5 it is 6.9p, and at SCOP 4.0 it is 6.0p.
With these example prices the heat pump is a little dearer to run at SCOP 3.0, roughly level at 3.5 and cheaper beyond that. The ratio of electricity to gas price (here 4:1) is the key number: if your electricity is four times the price of your gas, a heat pump needs a SCOP above about 3.5 to match a boiler. Time-of-use tariffs that offer cheap overnight rates, and heat pump-specific tariffs, can shift the balance noticeably, because a heat pump with a hot water cylinder can store heat when electricity is cheapest. Solar PV panels can also offset some of the daytime running cost. You can test your own prices and efficiency in our heat pump COP and running cost calculator.
The Boiler Upgrade Scheme in 2026
The Boiler Upgrade Scheme (BUS), run by Ofgem, pays a grant towards replacing a fossil-fuel heating system with a heat pump or biomass boiler. It is open to homes in England and Wales; Scotland has its own support through Home Energy Scotland, and the Northern Ireland boiler scheme has closed. According to Ofgem, the current grant amounts are:
| Technology | Grant |
|---|---|
| Air-to-water, ground source or water source heat pump | £7,500 |
| Air-to-water or ground source heat pump in an eligible off-gas-grid property | £9,000, available until 31 March 2027 |
| Biomass boiler | £5,000 |
| Air-to-air heat pump (residential only) | £2,500 |
The grant is taken off your installer's quote as a discount: your MCS-accredited installer applies for the voucher on your behalf, so you do not pay the full amount and claim it back. Other secondary sources report that eligibility rules were relaxed during 2026, including dropping an EPC requirement, but the details and scheme end date should be checked on the Ofgem and GOV.UK pages before you commit. Grant levels and deadlines are policy decisions and can change.
Will a heat pump work in your house?
A heat pump works best in a house whose heat loss is moderate, so it can heat the rooms with water at around 35–50°C. Three factors decide that.
Heat loss and insulation
A professional MCS survey calculates the heat loss of each room and the total design heat load. A typical 1930s semi with some insulation might need somewhere in the region of 6–9 kW at design conditions; a well-insulated modern house far less. Done properly, the survey sizes the unit and finds how many radiators must be larger. Cheap measures first, such as loft insulation, draught-proofing and cavity wall insulation, can cut the load and the size of unit you need to buy.
Emitters
Heat pump flow temperatures are lower, so each radiator gives less heat than it does at boiler temperatures. Swapping some radiators for larger or double-panel models, or adding fan-assisted units, often solves it. Underfloor heating works particularly well with a heat pump because it runs at low temperatures. Read our radiator sizing guide to see how much output you lose when water temperatures drop.
Space
You need room outside for the outdoor unit with clearance for airflow, and typically a hot water cylinder indoors, since heat pumps do not usually work with combi arrangements. Many installations do not need planning permission in England if they satisfy permitted development conditions, which include noise and distance criteria, but check with your local authority and see the Planning Portal. Scotland, Wales and Northern Ireland have their own rules.
Upfront cost
Costs vary hugely with the house, so use quotes rather than averages. A like-for-like replacement gas combi boiler in the UK usually costs a small fraction of a complete heat pump installation. A heat pump job commonly involves the unit, a cylinder, controls, pipework changes and sometimes new radiators. The BUS grant removes a substantial chunk, but a net outlay above that of a boiler is common. Get at least three MCS-accredited quotes, and check what each includes: cylinder size, radiator upgrades, and commissioning. Zero-rated VAT on installed energy-saving materials, including heat pumps, applies in Great Britain until 31 March 2027, after which VAT on installation is due to return to 5%.
Comfort, noise and day-to-day living
Heat pumps heat steadily and more gently. Rather than blasting a house up to temperature quickly and switching off, a well-set heat pump runs for long periods at lower power, so radiators feel warm rather than hot. Many householders find rooms more even in temperature; some dislike that they cannot get a sudden boost. Outdoor units make a low fan noise; installers should position the unit away from neighbours' windows and bedrooms, and the quoted sound level at a given distance is worth asking for. The heat pump also needs defrost cycles in cold, humid weather, which are normal.
Hot water, cylinders and legionella settings
A heat pump normally heats a stored cylinder rather than producing hot water on demand. The cylinder is typically heated to around 50–55°C for daily use, and many systems run a periodic higher-temperature cycle to guard against legionella bacteria, usually using an immersion element. That cycle uses more electricity, so ask the installer how often it runs by default and whether it can be scheduled for cheap-rate periods. If your house currently relies on a combi boiler, you will gain a cylinder but lose a cupboard or airing-space; measure the space early, because a cylinder for a family of four is considerably larger than the old boiler. Unvented cylinders are common, and they carry their own installation and annual-inspection expectations, which a competent installer will explain.
Maintenance and lifespan
Heat pumps need an annual service, as boilers do, but servicing is largely checks of refrigerant circuit, filters, water quality and controls. Manufacturers commonly quote lifespans of fifteen to twenty years for well-maintained units, while a gas boiler is often reckoned at ten to fifteen years; these are industry rules of thumb rather than guarantees, so check the warranty terms offered on your specific unit. Because heat pumps use refrigerants regulated under environmental rules, repair and servicing should be done by suitably qualified engineers. Keep the area around the outdoor unit clear of leaves, snow drifts and stored items, and keep a record of commissioning data, which is useful for any future warranty claim.
When the boiler still wins
- A small flat with no outdoor space or cylinder space.
- A very leaky, solid-walled house where fabric improvements are not feasible and radiator replacement would be extensive.
- A short time horizon: if you will move within a couple of years, a lower upfront cost may be a priority.
- An older boiler working well, where the grant does not cover the extra spend.
Heating policy is also shifting. Gas boilers remain legal to install in existing homes, but new homes are moving away from fossil-fuel heating under the Future Homes Standard, which is one reason installer capacity and product choice for heat pumps are growing. That is background rather than a deadline for you.
A practical decision process
- Find out your current annual gas use from bills, and your annual electricity use.
- Do the simple fabric improvements that cost little.
- Ask two or three MCS installers for a room-by-room heat loss calculation and a proposed system with the expected SCOP and running cost.
- Compare the net cost after the BUS voucher and the estimated annual bills with your gas boiler alternative.
- Check whether you can use a time-of-use tariff, and whether solar panels (see our solar panel guide) might lower the daytime cost.
- Check the installer's MCS certification and consumer code membership before signing.
Quoted SCOP figures are design estimates. Ask to see how they were calculated and whether they assume flow temperatures your radiators can realistically deliver.
This is general information, not professional advice. Prices and grants change, so confirm them with Ofgem, GOV.UK and your installer.