Combi boiler replacement
A combi boiler has no hot water cylinder. It heats your radiators and your hot water straight from the boiler, when you need it, according to the Energy Saving Trust [S1].
Who a combi suits
The Energy Saving Trust says [S1]:
- a combi is good where space is short, and is often the cheapest type;
- it is better for smaller households;
- it is less suited to homes with several bathrooms or high hot water demand.
Checkatrade adds that combis can struggle with old pipework or low water pressure [S2]. If your home uses a lot of hot water at once, read about system boilers.
Thinking about a heat pump later? The Energy Saving Trust says most heat pumps work with a hot water cylinder, so switching to a combi now may mean fitting a cylinder again later [S1].
What a combi costs
From Checkatrade's UK cost guide (price tables updated March 2026, read 30 September 2026) [S2]:
| Job (supply and install) | Checkatrade figure |
|---|---|
| Like-for-like combi replacement, same place | £2,500 to £4,000 |
| Gas combi, average UK total | about £3,000 |
| System boiler to combi | about £4,000 |
| Regular boiler to combi | about £4,000 |
| Back boiler to combi | about £4,000 |
Checkatrade's gas combi row gives £650 to £3,000 for the boiler and £1,000 to £2,000 for the installation [S2].
How long it takes. About 4 to 6 hours for a like-for-like swap, about 1.5 to 2 days to change a regular boiler to a combi, and up to about 3 days to change a system boiler to a combi [S2].
These are UK ballpark figures, not Cheltenham prices, and not a quote. The cost guide lists the extras, such as a powerflush or moving the boiler.
The rules
- Gas Safe. The HSE says anyone paid to work on gas appliances in a home must be Gas Safe registered [S3]. Check the engineer's ID card and licence number: our FAQ says how.
- Building regulations. An installer registered with a competent person scheme can self-certify the work. gov.uk says they give you a certificate within 8 weeks of finishing [S4].
- Condensing boiler. The Planning Portal says a replacement gas boiler will probably have to be a condensing boiler, unless there is a good reason it cannot be [S5].
- Flue. Planning permission is not normally needed when the work is all inside. An outside flue is normally permitted development if it meets the conditions, but conservation areas and listed buildings have extra rules [S6]. See the area pages for Cheltenham, Gloucester and Tewkesbury.
Get a quote
Request a free quote. We are a quote-matching service, not an installer: with your consent we pass your enquiry to one Gas Safe registered boiler installer.
Frequently asked questions
How much does a new combi boiler cost?
Checkatrade's UK guide (updated March 2026) says a like-for-like combi replacement in the same place usually costs £2,500 to £4,000, supply and install. Its average UK total for a gas combi is about £3,000 [S2].
Is a combi boiler right for a big house?
Often not. The Energy Saving Trust says combis are better for smaller households and less suited to homes with several bathrooms or high hot water demand [S1].
How long does a combi swap take?
Checkatrade says a like-for-like replacement takes about 4 to 6 hours [S2].
Sources
- [S1] https://energysavingtrust.org.uk/advice/boilers/, accessed 2026-09-30
- [S2] https://www.checkatrade.com/blog/cost-guides/new-boiler-cost/, accessed 2026-09-30
- [S3] https://www.hse.gov.uk/gas/domestic/faqs.htm, accessed 2026-09-30
- [S4] https://www.gov.uk/building-regulations-approval/use-a-competent-person-scheme, accessed 2026-09-30
- [S5] https://www.planningportal.co.uk/permission/common-projects/boilers-and-heating/building-regulations, accessed 2026-09-30
- [S6] https://www.planningportal.co.uk/permission/common-projects/boilers-and-heating, accessed 2026-09-30