Comparison Guide · Updated May 2026
Back Boiler to Combi Conversion: Cost & Timing 2026 (UK)
Converting a gas back boiler to a combi in 2026 UK is £3,200–£5,400 like-for-like or £4,500–£7,500 with new pipework, magnetic filter, smart thermostat and chimney capping. Most jobs complete in 1–3 days. The biggest cost variable is whether the new combi can sit in the same airing cupboard or has to relocate to a kitchen wall — relocating adds £800–£1,400.
Like-for-like: £3.2k–£5.4k With upgrades: £4.5k–£7.5k Job time: 1–3 days
Side-by-side: back boiler vs new combi (2026)
| Existing back boiler | New 2026 combi | |
|---|---|---|
| Typical efficiency | 62–72% | 90–94% (ErP A) |
| Annual gas use (12,000 kWh demand) | ~17,500 kWh | ~13,333 kWh |
| Annual gas bill at 7.5p/kWh | £1,310 | £1,000 |
| Hot water | Slow reheat tank | Instant, mains pressure |
| Floor space freed | — | Airing cupboard + loft tank |
| Warranty | Expired | 10 years (Worcester / Vaillant / Viessmann) |
What’s in a 2026 back-boiler-to-combi quote
- New combi boiler: £1,400–£2,400 (24–30 kW typical 3-bed)
- Strip out of back boiler + fire front: £200–£400
- Strip out of cold water tank + hot water cylinder + F&E tank: £300–£500
- Chimney cap + ventilator: £120–£220
- New flue: £180–£350
- Magnetic system filter (Adey / Fernox): £180–£260 fitted
- Power flush: £380–£650 (essential on 20+ year old systems)
- Smart thermostat (Hive / Nest / Tado): £180–£300 fitted
- Make-good plastering / brickwork where the fire was: £200–£600
- Labour (2 Gas Safe engineers, 1–3 days): £900–£1,800
Timing — what happens day-by-day
- Day 1 (morning): Isolate gas & water, drain system, remove fire front and back boiler, cap chimney
- Day 1 (afternoon): Run new flue, install combi on its wall, run new gas (15 mm or 22 mm) and water connections
- Day 2: Run mains pressure cold to taps, remove cold tank from loft, remove hot cylinder, power flush system, fit magnetic filter
- Day 3 (if needed): Make good plaster & redecorate fireplace alcove, commission & user handover, Benchmark certificate
Where the price moves
- Relocating the combi to a kitchen wall: +£800–£1,400 (new flue route, gas extension)
- Switching to a System / Heat-Only boiler instead of combi: add £1,200–£2,200 (keeps cylinder, useful for high-demand households)
- Bedroom-above-boiler relocation: some homeowners move the boiler to free up an airing cupboard — usually £500–£900 if pipework is straightforward
- Lead supply pipe upgrade: if your incoming mains is lead, it MUST be MDPE for combi mains pressure — add £400–£1,200
When NOT to convert to a combi
- Multiple bathrooms in regular simultaneous use — a combi’s flow rate (12–18 L/min) struggles. Choose a system boiler + unvented cylinder instead.
- Low incoming mains pressure (under 1 bar dynamic) — combi performance collapses; a Salamander pump or system boiler is the fix.
- You’re planning an ASHP within 5 years — reuse the existing cylinder space rather than rip-out-and-refit twice.
FAQs
For a single-bathroom household: yes — you save £250–£400/year on gas, get instant hot water, free up loft and cupboard space, and inherit a 10-year warranty. Payback is typically 8–14 years on gas savings alone, before reliability and resale value. For multi-bathroom households, a system boiler + unvented cylinder is the better answer.
Almost never — flues are Permitted Development unless your property is listed or in a conservation area where front-facing flues are restricted. Building Regulations notification is automatic via the Gas Safe registration.
Heating and hot water go off at the start of day 1 and typically come back on by the end of day 1 or mid-day 2. Most installers can fast-track to overnight heat the same day for vulnerable households.
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