Is it worth replacing an old boiler?

Replacing an old gas boiler with a new A-rated condensing boiler and better controls saves about £190 to £350 a year in a semi-detached house, depending on how inefficient the old one is, according to the Energy Saving Trust. In a detached house replacing a G-rated boiler it is about £500; in a mid-floor flat £90 to £170.

Energy Saving Trust estimates at July 2026 fuel prices, for a gas heated home fitting an A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator valves in place of an older boiler with a programmer and room thermostat. The rating is the old boiler's seasonal efficiency band. The Trust publishes these for gas only and gives no end-terrace figure. These are modelled savings, which run higher than what homes have been measured to save, below.

An old non-condensing boiler usually sits somewhere in those bands, and the oldest in G. Part of the saving comes from the thermostatic radiator valves, which let each room be kept at its own temperature.

At the Trust's typical cost of £3,500 for the job, a semi-detached house pays back in about 10 years replacing a G-rated boiler and about 18 years replacing a D-rated one. A detached house with a G-rated boiler pays back in about 7 years; a mid-floor flat with a D-rated boiler takes about 39 years.

The Energy Saving Trust's £3,500 typical cost of replacing a G-rated gas boiler with an A-rated one with thermostatic radiator valves, divided by its yearly saving.

If the old boiler has already failed, the sum changes: you are paying for a new heating system anyway, and the saving is a bonus on top. Our new boiler cost calculator works out the cost range, saving and payback for your home and boiler.

Government measurements of real homes show smaller savings. Across homes in England and Wales that had a new condensing boiler fitted, gas use fell by a median 2.3%, about £20 a year for a typical 3 bed semi. Where new heating controls went in at the same time it fell by 11.5%, about £101 a year.

The measured figures cover every boiler replaced, whatever it replaced, including boilers that were already condensing, and many homes take part of the gain as warmth rather than a lower bill. The Trust's figures model a specific old boiler being replaced. For a genuinely old, non-condensing boiler, expect something between the two, and note how much the controls add.

DESNZ National Energy Efficiency Data-Framework, impact of measures 2026: median change in gas use in homes that had the measure fitted between mid-May 2023 and mid-May 2024, priced on 11,000 kWh of gas a year for an average 3 bed semi at 7.97p per kWh, the same figures as our ECO4 guide.

The Energy Saving Trust says your boiler is a condensing boiler if the flue is plastic, if a plastic pipe runs from the bottom of the boiler through the wall to a drain, or if it is a gas or LPG boiler fitted after 2005 or an oil boiler fitted after 2007. A metal flue usually means it is not.

A condensing boiler recovers heat from the flue gases that an older boiler sends up the flue, which is why replacing a non-condensing boiler saves far more than replacing one condensing boiler with a newer one.

It pays when the boiler is old and non-condensing, especially a G-rated one in a bigger home: a detached house saves about £500 a year and pays back in about 7 years. It also makes sense when the boiler has failed, or keeps failing and repairs are adding up: our boiler repair or replace calculator sets a repair quote against the cost of a replacement.

It does not pay to replace a working condensing boiler for the fuel saving: the measured median for any new condensing boiler is £20 a year in a 3 bed semi, so a £3,500 job would never pay back on fuel. Nor does it pay in a small flat with a D-rated boiler, at about 39 years. Better controls on the existing boiler may be the cheaper step.

A new boiler also lifts an EPC: in our points model a modern condensing boiler adds about 8 points when it replaces one over 15 years old. Our guide to improving an EPC rating compares it with cheaper ways to gain points.

Not in existing homes, on what the government has set out on GOV.UK so far. It has set no date after which a gas boiler cannot be repaired or replaced in a home that already has one. The rule it has set is for new homes: in England the Future Homes Standard applies from 24 March 2027, with transitional arrangements, and homes built to it will have low carbon heating such as a heat pump.

Government grants already point the other way. The Boiler Upgrade Scheme pays £7,500 towards an air source or ground source heat pump and £5,000 towards a biomass boiler in England and Wales, but nothing towards a gas boiler.

A heat pump for an average 3 bed semi costs a median £12,700 installed, about £5,200 after the £7,500 grant, against about £3,500 for a new gas boiler. Whether it costs less over 15 years depends mostly on the electricity tariff.

Our boiler vs heat pump calculator compares the two over 15 years for your home, and heat pump vs new boiler explains when each wins.

• Energy Saving Trust, Boiler types explained (last updated 20 May 2026): yearly savings for a gas heated home fitting an A-rated condensing boiler with a programmer, room thermostat and thermostatic radiator valves in place of an older boiler, by the old boiler's rating, at July 2026 fuel prices; £3,500 typical cost of replacing a G-rated gas boiler. Checked 5 October 2026.
• Department for Energy Security and Net Zero, NEED impact of measures data tables 2026: the median fall in gas use in homes that had a new condensing boiler fitted, with and without new heating controls, priced at the Ofgem cap gas rate of 7.97p per kWh.
• Ofgem, energy price cap for 1 October to 31 December 2026: gas 7.97p per kWh by direct debit.
• GOV.UK, Apply for the Boiler Upgrade Scheme: grants for air source and ground source heat pumps, biomass boilers and air-to-air heat pumps, checked 5 October 2026.
• Ministry of Housing, Communities and Local Government, Building Circular 01/2026: the Future Homes and Buildings Standards.
• Heat pump cost from our heat model, js/heat-model.js: the median installed price under the Boiler Upgrade Scheme for a heat pump of the size the home needs (DESNZ statistics, August 2026). EPC points from our EPC calculator model.