System boiler replacement
A system boiler heats your radiators and stores hot water in a separate cylinder, according to the Energy Saving Trust [S1].
Who a system boiler suits
The Energy Saving Trust says [S1]:
- it suits larger families with higher hot water needs, because several taps can run at once;
- the cylinder can also store energy from solar panels;
- heat is lost from the cylinder over time.
Checkatrade adds that system boilers are larger than combis and give a limited amount of hot water [S2]. Smaller household with little space? Read about combi boilers.
Thinking about a heat pump later? The Energy Saving Trust says most heat pumps work with a hot water cylinder [S1]. A system boiler already has one.
What a system boiler costs
From Checkatrade's UK cost guide (price tables updated March 2026, read 30 September 2026) [S2]:
| Job | Checkatrade figure |
|---|---|
| Upgrading to a newer system boiler (supply and install) | about £3,000 |
| Average gas system boiler (supply only) | about £1,500 |
| System boiler to combi (supply and install) | about £4,000 |
A system to combi change can take up to about 3 days [S2]. These are UK ballpark figures, not Cheltenham prices, and not a quote. The cost guide lists the extras, such as a powerflush, a system filter or new controls.
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 system boiler cost?
Checkatrade's UK guide (updated March 2026) puts upgrading to a newer system boiler at about £3,000, supply and install. It gives about £1,500 for an average gas system boiler on its own, supply only [S2].
Who is a system boiler best for?
The Energy Saving Trust says system boilers suit larger families with higher hot water needs, because several taps can run at once [S1].
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