Back Boiler Replacement Cost & Options 2026 (UK)
Replacing a back boiler costs £2,500–£4,500 in 2026 once you include removing the old unit from behind the fireplace and fitting a modern combi or system boiler in a new location. Back boilers — the units hidden behind a gas fire, like the Baxi Bermuda — are decades old, inefficient and no longer made, so replacement rather than repair is almost always the right call. Here are your options, the costs, and what to expect on the day.
2026 back boiler replacement costs
| Option | Typical price (2026) | Time |
|---|---|---|
| Remove back boiler + fit combi (relocated) | £2,500–£4,000 | 2–3 days |
| Remove back boiler + fit system boiler + cylinder | £3,200–£4,500 | 2–3 days |
| Removal & make-good of fireplace opening only | £400–£900 | 0.5–1 day |
| Builder's work / plastering after removal | +£200–£600 | +0.5–1 day |
| Power flush of old system | +£400–£700 | +0.5 day |
Your replacement options
Almost everyone replacing a back boiler moves to a wall-mounted combi in the kitchen, utility or airing cupboard — it removes the fire surround unit, frees up the chimney breast and gives instant hot water. If you have two or more bathrooms, a system boiler with an unvented cylinder is the better fit. The old gas fire is usually removed at the same time and the opening made good or fitted with a decorative surround.
Why back boilers are being retired
Back boilers date from the 1960s–1980s, run at around 55–65% efficiency against 90%+ for a modern condensing boiler, and are no longer manufactured, so spare parts are scarce. Keeping one going wastes money on gas and risks being left without heat when a part finally fails. Replacing it typically cuts heating bills noticeably and comes with a long manufacturer warranty.
What to expect on the day
Removing a back boiler is more involved than a standard swap because the unit sits behind the fireplace and the gas fire has to come out first. Expect two to three days: strip-out and make-good, then fitting the new boiler, pipework and flue in its new position, followed by a system flush and commissioning. A dust sheet and some redecoration around the old opening are normal.
Back boiler replacement costs at a glance — 2026
Source: BestBuilders 2026 regional cost model · 519 UK towns
Full 2026 rate card
| Item | Typical cost (2026) | Notes |
|---|---|---|
| Back boiler out, combi in | £2,500–£3,800 | The usual route; one bathroom homes |
| Back boiler out, system + unvented cylinder | £3,200–£4,500 | Better for 2+ bathrooms |
| Decommission only (leave in situ, cap off) | £150–£400 | Legal if made safe; removal can wait |
| Strip out fire + back boiler unit | £250–£600 | Heavy, awkward, two-person job |
| Brick up and plaster the opening | +£300–£800 | Or keep it for an electric/log-effect fire |
| Powerflush the old pipework | +£300–£700 | Decades of sludge — usually needed |
| Upsized gas run to the new boiler | +£150–£450 | Old 15mm runs often undersized |
| Radiator replacements | +£150–£400 each | Only where corroded — not automatic |
Regional spread applies as with any heating work: London and the South East 15–25% above these ranges, the North and South Wales up to 15% below. See the boiler replacement cost guide for the full picture.
Why replacement beats repair every time
Back boilers (the Baxi Bermuda generation behind the living-room fire) run at roughly 60–70% efficiency against 90%+ for a modern condensing boiler — on a typical 3-bed gas bill that gap is worth £200–£400 a year. Production ended years ago, parts are salvage-only, and any work on the fire front carries modern gas-safety requirements the old installations rarely meet. Every pound spent repairing one is a pound towards nothing.
How the conversion runs (2–3 days)
- Day 1: drain down, strip the fire surround, remove the fire and back boiler unit, cap the gas.
- Day 1–2: run new pipework to the boiler position (kitchen or airing cupboard), fit the flue, upsize the gas run if needed.
- Day 2: hang the new boiler, powerflush the system, fit the magnetic filter and controls.
- Day 2–3: commission, register the warranty and Building Regs, make good the fireplace opening.
Choosing the installer: checklist
- Gas Safe registration checked on the register — ask for the card number and verify it.
- Has done back-boiler conversions before — ask specifically; the strip-out is the awkward part.
- Quote itemises the flush, filter, controls, making good and waste removal.
- Warranty registered by the installer, not left to you.
- Three itemised quotes — conversion prices vary more than straight swaps because the strip-out effort varies.
Red flags
- A quote matching a straight combi swap price — the strip-out and new pipe runs are real costs; something is excluded.
- No powerflush on a 30-year-old system.
- Leaving the old unit connected “for now” without capping and certifying it safe.
FAQs
Get 3 Free Back Boiler Replacement Quotes
BestBuilders matches you with up to 3 vetted installers experienced in back boiler removals. Compare quotes — no cost, no obligation.