What changes
A conventional (regular or heat-only) system has a boiler, a hot water cylinder, and tanks in the loft. A combi heats water on demand directly from the mains, so the cylinder and tanks are removed. That frees up space and removes the risk of loft tank leaks or freezing.
Costs
Converting to a combi usually costs £3,200–£5,500, more than a like-for-like swap because of the extra pipework, removal of tanks and cylinder, and possible gas pipe upgrades.
| Job | Typical cost |
|---|---|
| Conversion to combi (including removal of cylinder and tanks) | £3,200 – £5,500 |
| Power flush (if needed) | £400 – £700 |
| Gas pipe upgrade | £200 – £600 |
Indicative costs (2026)
Will a combi work for you?
Your installer should measure the mains flow rate before recommending a combi. See combi boiler hot water flow rate.
- Mains water pressure and flow rate must be adequate
- Best for homes with one bathroom
- Two people showering at once can be a problem
- Old pipework may struggle with mains pressure
- Power showers and some pumps need replacing
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Pros and cons
- Pro: frees up airing cupboard and loft space
- Pro: hot water on demand, no waiting
- Pro: mains-pressure showers
- Con: limited flow to several taps at once
- Con: no immersion heater backup
- Con: less suited if you plan to fit a heat pump later
Getting it done
A conversion usually takes two to three days. Get quotes from Gas Safe registered boiler installers, and see the cost to replace a gas boiler.
