Is converting to a combi right for you?
A combi conversion replaces a system or regular boiler, its hot water cylinder and any loft tanks with a single combi boiler that heats water on demand. It typically costs £3,000 to £4,200 fitted (checked October 2026) and takes one to two days. You gain cupboard and loft space, and you stop paying to keep a tank of water hot.
It works well for a lot of homes, but not all. Before anyone quotes, run through these three checks.
- How many bathrooms are used at once? A combi gives one strong flow. If two showers often run at the same time in your house, a combi will disappoint. Stay with stored hot water.
- How good is your mains water? A combi’s hot water can only be as good as the supply from the street. Your installer should measure flow rate and pressure at the kitchen tap. If the flow is weak, a conversion will not give you a better shower.
- How old is the pipework? Converting turns an open, low pressure heating circuit into a sealed one at around 1 to 1.5 bar. Old joints and valves can weep under that pressure.
If all three come back fine, a conversion is usually a good investment for a one bathroom home.
What the job involves
| Work | Why it is needed |
|---|---|
| Remove the hot water cylinder | A combi heats water directly, so no store is needed |
| Remove the cold water and feed tanks from the loft | The system becomes sealed and fed from the mains |
| Cap and reroute pipework | Hot and cold supplies now come from the boiler and mains |
| Fit the combi, often in a new position | Many old boilers sit where a combi will not fit or flue |
| Upgrade the gas pipe if needed | A combi needs more gas when heating water |
| Flush and filter the system | Old systems carry sludge that damages a new boiler |
The gas pipe point catches people out. A combi draws a lot of gas when you run a hot tap, and older 15mm gas pipes often cannot keep up. Upsizing to 22mm typically adds £150 to £400.
Sizing the new combi
A combi is sized on hot water flow, not house size. Most one bathroom homes suit 24kW to 30kW. Bigger combis give more flow, but only if the mains can supply it. Our guide what size boiler do I need? explains the numbers.
What changes in daily life
You get instant hot water at mains pressure, with no waiting for a cylinder to reheat. You lose the ability to fit an electric immersion heater as a backup, so if the boiler breaks down you have no hot water until it is fixed. You also lose the gravity-fed supply that some older showers and mixer taps were designed around, so check that your existing shower valve is suitable for mains pressure.
When to think twice
If you might want a heat pump in the next few years, keep the cylinder. Heat pumps need stored hot water, so removing a good cylinder now can mean paying to fit a new one later.
That does not mean you should rule out a combi. It means weighing the boiler you need today against where your heating might go. If a heat pump could be on the cards, compare it on our heat pump installation page first.
Before you sign
Ask each installer to state the combi model and output, whether the gas pipe is being upgraded, whether the tanks and cylinder are removed and taken away, and whether making good (filling holes and capping pipes neatly) is included. Check their Gas Safe ID and confirm it on the Gas Safe Register. City Boiler connects you with independent installers who are Gas Safe registered.
If you decide to keep stored hot water, see system boiler replacement or conventional boiler replacement. If you already have a combi, a like-for-like combi replacement is a simpler job.
Common questions
How much does it cost to convert to a combi boiler?
Converting from a system or regular boiler to a combi typically costs £3,000 to £4,200 fitted, checked October 2026. The price covers removing the cylinder and loft tanks, rerouting pipework and fitting the new boiler.
How long does a combi conversion take?
Usually one to two days. Removing tanks from a loft and capping off old pipework takes longer than a straight swap, especially if the boiler also moves position.
Will my old radiators cope with a combi?
Most will. A combi runs the radiators as a sealed, pressurised system, so weak joints or old valves that were fine on a tank-fed system can start to weep. A good installer checks the system before quoting and flushes it during the job.
What can I do with the airing cupboard space?
Once the cylinder is gone, the cupboard is free for storage or shelving. Some people keep a small part of it as a drying space, since a cylinder used to keep it warm and that heat will be gone.