Converting to a combi boiler means removing the hot water cylinder and the cold water tank in the loft, and heating water on demand instead. It is one of the most common boiler changes we are asked about — here is what it really involves.
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What a combi conversion changes
A conventional system stores hot water in a cylinder and feeds it from a cold water tank in the loft. A combi does away with both: it heats water straight from the mains the moment a tap or shower opens. The conversion means removing the cylinder and loft tank, capping or removing their pipework, and reconfiguring the boiler connections so the mains feeds the boiler directly.
The result is more space — the airing cupboard and the loft tank are gone — and hot water that never runs out. The trade-off is that a combi can only heat so much water at once, so two showers running together can stretch it.
Who a conversion suits
A combi conversion suits many smaller and mid-sized homes where one hot tap or shower runs at a time, and where the incoming mains pressure and flow are decent. It suits households who want the cupboard space back, or whose loft tank is old and they would rather see it gone.
It is less suited to larger homes with two or more bathrooms in regular simultaneous use, or where the mains flow is weak. Our combi vs conventional boiler guide compares the two ways of living with hot water, and the survey settles the question for your house.
What the work involves
Draining down the heating system and the stored hot water
Removing the old boiler, cylinder and loft tank
Running a mains cold feed to the new boiler and reworking the pipework
Fitting the new combi, flue and condensate route
Filling, commissioning and checking every tap and radiator
What affects the cost
A conversion is more work than a straight combi swap because of the removal and pipework changes. The boiler choice, the flue route, whether the gas supply pipe needs upgrading and how much making good is needed all shape the price. There is no honest one-size figure — the free survey and written quote give you the real one for your home.
Converting from oil to gas
A different kind of conversion altogether: if your home has an oil boiler and mains gas has reached your street, switching involves a new gas connection from the network as well as the boiler change. It is worth discussing early, as the gas connection has its own timeline. For homes staying off the gas grid, our LPG boiler installation and oil pages cover the alternatives.
Start with a survey
Chris will check your mains pressure and flow, look at the existing system and tell you honestly whether a conversion suits your household. Read about combi boiler installation, then use the new boiler quote page to arrange the visit.
Common questions
Combi Conversion FAQs
Clear answers before you arrange a visit.
How much does a combi boiler conversion cost?+
More than a straight swap, because the cylinder and loft tank come out and the pipework changes. The boiler, flue, gas pipe and making good all shape the figure. The survey and quote are free.
How long does a combi conversion take?+
Longer than a like-for-like swap because of the removal work. Chris will confirm the expected timescale for your home when he quotes.
Will a combi cope with two bathrooms?+
It depends on your mains flow and how the bathrooms are used. If two showers run at the same time most mornings, a system boiler with a cylinder may serve you better — the survey checks this properly.
Do I lose my immersion heater backup with a combi?+
Yes — the cylinder and its immersion go. If boiler breakdown backup matters to you, that is worth weighing up, and Chris will talk it through honestly.
