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01 · Ask
Repair, replace or upgrade?
Cold rads, no hot water, F22 fault code? Describe what's happening — the AI tells you if it's a £150 repair or a £3k replacement.
Replace your back boiler.
Old gas back-boiler swapped for a modern combi or system — full conversion, including the gas-fire opening.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
20,000+UK homeowners
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Hire a boiler engineer. Without the warranty void.
Read the brief on sizing, brands and warranty rules — then let Three local engineers quote on the right boiler for your house.
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02 · Read the brief
Walk in informed
kW sizing, modulation ratios, Benchmark cert, manufacturer warranty terms. Know what to ask before they recommend the most expensive box on the van.
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03 · Compare
Compare 3 engineer quotes
Three local engineers come back with itemised quotes — boiler, flue, controls, power-flush, commissioning — so the like-for-like vs upgrade choice is yours.
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Top tips
Hiring a boiler engineer, without the regret.
Four moves that separate a smooth job from a nightmare.
Get a Gas Safe ID before letting them in.
Boilers are gas appliances. Anyone working on yours must be Gas Safe registered — and on the right scheme for your fuel (LPG, oil, natural gas are separate).
Demand the Benchmark certificate.
Every new boiler comes with a Benchmark commissioning checklist. Without it, the manufacturer warranty is void — and the certificate goes in your seller's pack later.
Size the boiler to the radiators, not the house.
Oversized combis short-cycle, wear out fast and cost more to run. A heat-loss calc — not a 'same as before' swap — is the sign of a careful installer.
Power-flush before fitting.
On a 10-year-old system, sludge will write off your new boiler's heat exchanger in 18 months. A power-flush should be quoted; if it's not, ask why.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Boiler service (annual)
£80–£150
Combi boiler swap (like-for-like)
£1.8k–£3k
Combi boiler upgrade (premium)Vaillant/Viessmann tier
£2.8k–£4.5k
System boiler + cylinder
£3k–£5.5k
Power flush
£450–£900
Conversion from back-boilerInc. removal + replumb
£3.5k–£6k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
Infographic · VideoThe Boiler Engineer briefing.
Infographic
Boiler Engineer quick-view
Video guide
Boiler Engineer explained
Back boilers — installed behind a fireplace, common in 1960s–80s UK homes — are now obsolete and inefficient. Replacing one is more involved than a standard boiler swap because of the fireplace remediation. This guide covers what back boiler replacement actually costs in 2026 and what's involved.
What Does the Work Involve?
- Survey of existing fireplace, gas supply, flue.
- Drain down and disconnect existing back boiler.
- Remove back boiler from fireplace opening.
- Decommission and seal old chimney flue.
- Install new boiler in chosen location (often relocated).
- Power flush old system.
- Replace or board up fireplace opening.
- Commission new system.
Typical Costs
| Configuration | Low £ | High £ | Notes |
|---|---|---|---|
| Back boiler removal + combi install | 3,500 | 5,500 | Common conversion |
| Back boiler + system boiler install | 4,500 | 6,500 | If cylinder needed |
| Including fireplace remediation | +200 | +800 | Boarding/plastering |
| Including chimney cap/seal | +300 | +800 | Sometimes needed |
How Long Does It Take?
- 2–3 days for removal + new install.
- Plus 1–2 days for fireplace remediation if needed.
UK Regulations
- Gas Safe + Building Control as standard.
- Chimney sealing if no longer in use.
Common Problems
- Existing pipework asbestos — older homes; requires specialist removal.
- Discovery of additional damage — hidden corrosion.
- Fireplace structural issues — discovered when removing.
- Old gas pipework — may need full upgrade.
Frequently Asked Questions
Are back boilers safe?
Modern serviced ones yes. But they're 50%+ less efficient than modern A-rated boilers. Replacement is the priority.
Can I keep the gas fire?
Often yes if heating function moved to new boiler. Decorative gas fire only.
What if asbestos is found?
Licensed removal — typically £400-£1,500 additional. Standard for 1960s-80s back boiler installs.
Will efficiency improve significantly?
Yes — typically 30-40% reduction in gas bills moving from back boiler to modern A-rated.
Can I claim grants for replacement?
Eligibility depends on income/property. ECO scheme has provisions. Check with installer.
Should I have a system or combi?
Depends on bathroom count and existing pipework. Discuss with installer at survey.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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