Combi conversions
Lose the tanks, keep the hot water.
A combi conversion swaps an older tank-fed system (a regular or system boiler, a hot water cylinder and usually a cold tank in the loft) for a single combi boiler. You get hot water straight from the mains and an empty airing cupboard. It is a bigger job than a straight boiler swap, so we survey first and give you a fixed quote before anything is booked.
Who a combi conversion suits
It is a good fit for flats, terraces and semis with one main bathroom, where the airing cupboard space would be more useful as storage or a bigger bathroom.
It is a poor fit for a big house with two or three bathrooms used at the same time. A combi heats water as it flows, so it cannot fill a bath and run a shower at full pace together. For those homes we usually recommend a system boiler with an unvented cylinder instead, which still gets rid of the loft tank.
Check your water pressure first
On a tank system your shower runs off gravity. On a combi it runs off the mains. If your mains flow is weak, a combi could actually give you a worse shower than the one you have now.
We measure your flow rate at the kitchen tap on the survey. That one reading decides more about whether a conversion makes sense than anything else.
What a conversion involves
We drain the system, take out the old boiler, cylinder, cold water tank and the small feed and expansion tank, then cap or reroute the old pipes. The new combi goes on, usually in the kitchen or the old airing cupboard if a flue can reach outside from there.
Older systems are often dirty inside, and a new boiler hates sludge. We clean the system and fit a magnetic filter as standard, and suggest a power flush where the radiators show it is needed.
A few things that catch people out
Some older showers, especially gravity-fed power showers with their own pump, will not work on a combi and need replacing with a mains-pressure shower.
Old pipework and joints that coped happily with low tank pressure can weep once they are under mains pressure. We pressure test the system and tell you if anything needs replacing before we commit to a price.
What happens, step by step
Survey
We test your water pressure and flow, check the old system and agree where the combi will go.
Fixed quote
One written price, including any shower or pipe changes the conversion needs.
Conversion
Usually two to three days, with heating and hot water off for part of that time.
Handover
We commission the boiler, show you the controls and register the warranty.
What is included
- Survey including a mains pressure and flow test
- Removal of the old boiler, hot water cylinder and loft tanks
- New combi boiler fitted by a Gas Safe registered engineer
- Pipework capped or rerouted, with a full pressure test
- System clean and magnetic filter
- Controls set up and explained
Combi conversions FAQs
How much does a combi conversion cost?
Which? puts the extra labour and re-piping for going from a heat-only or system boiler to a combi at £400 to £800, on top of the boiler itself. Our new boiler cost page has the wider ranges. We give you a fixed price after the survey.
How long does a combi conversion take?
Usually two to three days. Removing the tanks and cylinder is straightforward; the time goes on pipework and getting the new flue in the right place.
Will my shower still work?
A normal mixer shower usually works well on a combi with decent mains flow. Electric showers are unaffected. Pumped power showers fed from a tank do not work on a combi and need swapping for a mains-pressure model, and we include that in the quote if it applies.
What happens to the airing cupboard?
It becomes yours. Many people put shelves in. Others take the wall out and use the space to enlarge the bathroom.
Is a combi conversion worth it?
For a one-bathroom home with good mains pressure, usually yes. You lose the tanks and stop heating a cylinder of water you might not use. In a busy home with several bathrooms, we would steer you to a system boiler instead.
Get a fixed price for your home
Tell us about the job and we will arrange a free survey.