Converting to a Combi Boiler: What Is Involved

If your hot water cylinder takes up a whole airing cupboard and you're tired of waiting for the tank to reheat, converting to a combi boiler is probably on your mind. It's one of the most common upgrades we quote for across the Midlands, and it can genuinely transform how a home runs day to day. But it's a bigger job than swapping one boiler for another of the same type, so it's worth knowing what's actually involved before you get quotes.

What changes when you go combi

A combi boiler heats water on demand, straight from the mains, so there's no need for a hot water cylinder or a cold water tank in the loft. If you're not sure how that differs from what you have now, our combi vs system vs regular boilers guide breaks it down properly. The short version: converting means removing equipment, not just installing it.

The work involved

A typical conversion includes:

• Removing the old cylinder, feed and expansion tanks, and any redundant pipework
• Making good the airing cupboard and loft space (this is often a nice bonus for storage)
• Rerouting pipework to run hot and cold water directly from the new combi
• Upgrading the gas supply pipe if the new boiler needs more capacity
• Fitting the combi itself, usually on an external wall for the flue
• Pressure testing the system and checking mains water flow rate

Because there's more pipework and disruption than a straight swap, it usually takes longer. Our guide on how long a boiler installation takes covers realistic timeframes, but for a conversion you should generally expect two to three days rather than one.

Is your home suitable?

Combi boilers rely on decent mains water pressure and flow rate. If your incoming supply is weak, several taps or showers running at once will suffer. A good engineer checks this before quoting, not after fitting. The Energy Saving Trust has useful independent guidance on boiler types and efficiency if you want a broader view before deciding.

Getting the size right matters too. Too small and you'll be waiting for hot water or losing pressure in the shower; too large and you're paying for capacity you don't need. Our boiler sizing guide explains how engineers calculate this properly rather than guessing.

What it costs

Conversions cost more than like-for-like swaps because of the extra pipework, tank removal, and making good. Our new boiler cost guide has current price ranges, and it's worth knowing that national suppliers often quote £3,000 or more for a straightforward combi swap, so a conversion with additional work can sit well above that. Spreading the cost is common; see our page on boiler finance if that's useful.

Choosing the right boiler and installer

Brand matters less than correct sizing and a clean installation, but if you're comparing options our Worcester Bosch vs Vaillant guide and best boiler brands compared are good starting points. Whoever you choose, confirm they're Gas Safe registered. You can check any engineer's licence number directly on the Gas Safe Register website, and it's worth reading independent reviews on Which? before committing to a specific model. All our engineers across the Midlands are Gas Safe registered and every job comes with a fixed price agreed upfront, so there are no surprises once the old cylinder is out and the walls are opened up.

Once it's fitted, keep it running well with annual servicing, and if you ever notice pressure dropping or strange noises, our guides on losing pressure and banging noises can help you work out what's going on.

Ready to see what a conversion would cost in your home? Get a fixed price and we'll take it from there.