Conversion
Converting to a combi boiler
A conversion costs more than a like-for-like swap because the cylinder, tanks, and old pipework all come out. The headline range is £3,000 to £5,000 supplied and fitted. The exact number depends on the boiler type you are leaving behind.
Headline costs
Conversion price by source type
From
Regular (heat-only)
£3,000 to £4,500
2 days
Most common conversion. Cylinder, header tank, and feed-and-expansion tank all out. Pipework rerouted.
From
Back boiler
£3,500 to £5,000
2 to 3 days
Builder's work to remove the boiler from behind the fireplace, hearth made good. Add £500 to £800 for the masonry side.
From
System (with cylinder)
£2,500 to £4,000
1.5 to 2 days
No loft tanks to remove, but the cylinder still comes out. Simpler than regular-to-combi, harder than like-for-like.
The work
What a conversion involves
Eight tasks on a regular-to-combi conversion. System and back-boiler conversions skip or extend a step or two.
- 01
Drain the central heating and disconnect the gas supply.
- 02
Remove the old boiler, the hot-water cylinder, and the loft tanks.
- 03
Cap or remove the old pipework that fed the cylinder and tanks.
- 04
Upgrade the gas supply pipe to 22 mm if the existing pipe cannot carry the new combi's input.
- 05
Reroute the central heating flow and return to the new combi position.
- 06
Mount the new combi, connect gas, water, and flue.
- 07
Powerflush the system to clear sludge from the old radiators.
- 08
Refill, pressurise, commission, and run a flue gas analysis. Hand over with the Benchmark certificate.
Pause first
Is converting the right call?
A conversion is permanent. Removing the cylinder is not easily reversed. Make sure a combi suits the household before you commit.
Convert if
- +You have one or two bathrooms with one user at a time
- +You want the loft and airing cupboard back
- +Mains pressure is good (over 1 bar)
- +You are happy to run hot taps one at a time
Stay with a cylinder if
- -You have three or more bathrooms used in parallel
- -Mains pressure is consistently low
- -You run a soak-tub or rainfall shower regularly
- -You like the immersion-heater backup of a system boiler
FAQs
Common questions
How long does a regular-to-combi conversion take?v
Two days on a typical 3-bed house. Day one: drain, strip out, remove cylinder and tanks, upgrade gas pipe. Day two: fit new combi, reroute pipework, powerflush, commission, hand over. Back-boiler conversions stretch to 3 days because the masonry work takes time.
Do I need planning permission to convert?v
No. Conversions are notifiable through Gas Safe, not planning. The fitter notifies the local authority on your behalf under Building Regulations Part J (Combustion Appliances).
Will my radiators still work after a conversion?v
Yes, the radiators stay. The conversion changes how hot water is supplied to them, not the radiators themselves. Old radiators may benefit from a powerflush at the same time, which is included on most conversion quotes.
Can I convert back to a regular boiler later?v
Technically yes, but it is expensive and rarely worth it. Putting the cylinder, tanks, and old pipework back is more work than the original conversion. Most homeowners who convert and regret it install a system boiler later, not a regular one.
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