Back Boiler Replacement: Your Options, Costs and Grants in 2026

Quick answer: A gas back boiler hidden behind your fire is typically only 70–80% efficient, against 90%+ for a modern condensing boiler. There are three main routes out: a full combi conversion (typically £3,500–£5,000 all-in), a system boiler with a hot water cylinder, or an air source heat pump with the £7,500 Boiler Upgrade Scheme grant.

If your heating comes from a unit built into the chimney breast behind the living-room gas fire, you have a back boiler — most likely a Baxi Bermuda, one of the most common models fitted in UK homes from the 1960s to the 1980s. They were brilliantly compact for their time, but their time has passed: production ended years ago, and it is no longer permissible to install a new non-condensing boiler in the UK. Here are your options in 2026, with realistic costs and the grants that actually apply.

Why replace a working back boiler?

• Efficiency. Most back boilers run at around 70–80% efficiency, so 20–30% of the gas you pay for goes up the chimney; a modern A-rated condensing boiler is rated at over 90% on the ErP efficiency scale. How much that saves depends on how much gas you burn; our boiler cost guide models it town by town (how we calculate costs).
• Parts and expertise are drying up. Repairs get slower and dearer every year as spares become scarce.
• Space. Removal frees the chimney breast, and a combi conversion frees the airing cupboard and loft tanks too.
• Your EPC. An ancient boiler drags down your EPC rating — which matters when selling or letting.

Option 1: Convert to a combi boiler

The most popular route. The back boiler and fire are stripped out, the opening is capped and made good, and a wall-mounted combi boiler goes in elsewhere (usually the kitchen), heating water on demand. The hot water cylinder and loft tanks go, freeing serious storage space. The trade-offs: it’s the most disruptive gas option — typically two to three days of work — and a single combi can struggle to serve two bathrooms running at once. Our combi vs system vs regular guide covers how to choose.

Option 2: A system boiler and cylinder

If your household gets through a lot of hot water — or you simply want to keep stored hot water — a system boiler paired with a modern cylinder is the natural fit. It reuses more of your existing pipework layout than a combi conversion, supports multiple bathrooms comfortably, and pairs well with solar panels via a cylinder immersion diverter later. Our guide to central heating system types explains how the pieces fit together.

Option 3: Skip gas entirely — fit a heat pump

A back-boiler home usually needs system-wide work whichever route you choose, which makes it a sensible moment to consider leaving gas behind. An air source heat pump qualifies for the Boiler Upgrade Scheme grant of £7,500 in England and Wales; your MCS-certified installer applies on your behalf (GOV.UK). Heat pump installations also carry 0% VAT until March 2027. Against our modelled typical cost of £10,684 for a heat pump, the grant brings the net cost to around £3,200 (our modelled estimate), in the same territory as a combi conversion. Some radiators may need upsizing to suit lower flow temperatures, but since a back-boiler swap involves major heating work anyway, the extra disruption is smaller than it would be in a home with a modern system. Our heat pump guide covers suitability.

One scoping note: until 31 March 2027 the grant rises by £1,500 to £9,000, but only for homes heated by oil or LPG that have no mains gas connection (applications from 21 July 2026). A gas back boiler doesn’t qualify for the uplift, so the standard £7,500 applies (GOV.UK).

Costs compared

Sources: Heatable and HeatingForce back boiler guides · checked July 2026; Home Energy Quotes modelled estimates. Our new boiler cost guide has the full breakdown of what moves gas boiler prices, and our cost calculator gives a figure for your region.

Grants that can help in 2026

• Boiler Upgrade Scheme: £7,500 towards an air or ground source heat pump in England and Wales, with funding confirmed to 2029/30. It also funds biomass boilers in rural homes off the gas grid, but never a new gas boiler.
• Warm Homes: Local Grant: free upgrades in England for homes rated EPC D to G where household income is £36,000 or less, someone gets a means-tested benefit, or the home is in an eligible postcode. Grants are capped at £16,600 for energy-efficiency work (including solar panels and batteries) plus £16,600 for low-carbon heating such as a heat pump, and the scheme runs to March 2028 through participating local councils (GOV.UK).
• ECO4: in its final months. Energy suppliers must finish installations by 31 December 2026 and no successor supplier obligation has been announced, so treat any ECO4 offer as a bonus rather than a plan (Ofgem).
• Scotland: the BUS covers England and Wales only; Scottish households should look at Home Energy Scotland grants and loans instead.

A safety warning about redundant back boilers

If a previous owner removed the fire but left the old back boiler unit sealed in the chimney breast, take this seriously: the Health and Safety Executive has warned that a redundant back boiler left in a sealed condition can explode if a fire is later lit in front of it, because residual water inside turns to steam with nowhere to go — incidents have caused serious injuries and one fatality. A decommissioned unit should be drained and removed, or at minimum left properly vented. If you’re not certain what’s behind your fireplace, don’t light a fire in it — have it inspected by a Gas Safe registered engineer first.

FAQs

Can I keep my back boiler and just repair it?

For now, sometimes — there’s no law forcing you to remove a working one. But parts are scarce, each repair buys time on a unit wasting a fifth or more of your gas, and a failed heat exchanger usually forces the decision anyway. Our guide to the signs a boiler needs replacing helps you judge the tipping point.

How long does a back boiler conversion take?

Typically two to three days for a full combi conversion — removal of the old unit, new pipework, flue, making good and commissioning. A heat pump installation on the same house is usually comparable once the system design is agreed.

Will removing it wreck my fireplace?

There’s making-good to do — the opening is capped and plastered, or fitted with an electric fire or a stove (a solid-fuel stove needs its own regulations sign-off). Most installers include basic making-good in the quote; redecorating the chimney breast is normally down to you, so ask for it as a line item.

Ready to price it up? Try our cost calculator for a regional estimate — then request a free quote and up to three approved installers who cover your area may contact you, with no charge and no obligation.