Updated 25 September 2026
Back boiler to combi cost UK 2026
The going rate for back boiler to combi in 2026 is £3,500–£6,000 (total, ex VAT). Below: a calculator, what the job involves, materials at three price levels, and the prep that changes the price.
Market band£3,500–£6,000
Worked example, 1 install£3,684
Time on site2.5 days
Back boiler to combi calculator
Price a real job freeUses your rates, your speed and your receipts.
Market band source: Heatable.
What the job involves
| Stage | Task | Typical time |
|---|---|---|
| out | Remove back boiler and gas fire | 8 h |
| out | Remove cylinder and loft tanks, cap off | 4 h |
| in | Re-route and cap pipework | 6 h |
| in | Fit boiler, flue and controls | 5 h |
| in | Power flush the system | 4 h |
| in | Close off and make good the fireplace | 3 h |
Materials at three price levels
| Material | Budget | Trade choice | Premium |
|---|---|---|---|
| Combi boiler (A-rated, 25–30 kW) each | £750 Entry combi, 7-year warranty — Lowest cost compliant boiler | £1,000 Mid-range combi, 10-year warranty — Reliable, parts easy to get | £1,500 Premium combi, 12-year warranty — Quieter, better hot water |
| Magnetic filter and cleanser each | £90 Basic magnetic filter — Catches sludge | £130 Branded filter and cleanser — Protects the heat exchanger | £180 Dual-action filter — Best protection |
| Flue kit each | £60 Standard horizontal flue — Straight out the wall | £90 Telescopic flue — Fits the wall thickness | £120 Vertical or plume management kit — Awkward terminal positions |
| Heating controls each | £40 Wired programmable stat — Meets Part L controls | £80 Wireless programmable stat — Place it anywhere | £180 Smart thermostat — App control |
| Valves, pipe, fittings and condensate allowance | £50 Standard fittings — Connects up | £80 Fittings plus frost-protected condensate — No frozen condensate | £120 Full copper tails and lagging — Tidy, efficient |
| Conversion pipework and caps allowance | £180 Plastic pipe and caps — Re-route and cap off | £300 Copper re-route — Durable | £450 Copper plus new isolation valves — Tidy conversion |
| Power flush chemicals set | £40 Standard cleanser — Shifts sludge | £60 Branded cleanser and neutraliser — Faster clean | £90 Heavy-duty descaler set — Badly sludged systems |
| Fireplace closure panel and vent set | £60 Board and vent — Closes the opening | £100 Board, vent and trim — Tidy finish | £150 Plastered closure — Ready to decorate |
Trade prices ex VAT, estimates checked 2026-09-25. Log your receipts in Grafto and your own prices replace these.
What changes the price
- Mains flow and pressure? good (tested) · poor · not tested big effect
- Controls? keep existing · new programmable stat · smart controls
Prep to look for
| Condition | Check | Effect |
|---|---|---|
| Sludge in the system | black water from a bleed valve, radiators cold at the bottom | Power flush the system |
| Possible asbestos lagging | old grey or white pipe lagging | Possible asbestos lagging: stop and test before disturbing |
| Microbore pipework | 8–10 mm pipes to radiators | Microbore: limits radiator upgrades and blocks easily — flush it |
Questions
How much does back boiler to combi cost in the UK in 2026?
The researched market band is £3,500–£6,000 (total, ex VAT), from Heatable. A worked example of 1 install comes to about £2,928–£4,942 across budget, trade and premium materials.
How long does back boiler to combi take?
The worked example of 1 install takes about 2.5 working days.
What changes the price of back boiler to combi?
Mainly mains flow and pressure, plus any prep found on site: sludge in the system, possible asbestos lagging, microbore pipework.