Combi vs system boiler
Supports revision for: C&G 6035 Unit 206. Coverage references only; no awarding-body affiliation or qualification is conferred.
A typical combi boiler heats tap water as it is requested and also serves central heating. A system boiler supplies the heating circuit and heats stored domestic hot water in a separate cylinder through a heat exchanger. Start with where the tap water is heated, not the size of the boiler case.
Plumbing learning · Sources checked 29 October 2026
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0How much hot water does a typical combination boiler store for the taps?
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| Feature | Typical combi | System boiler with cylinder |
|---|---|---|
| Hot-water storage | Usually no separate domestic hot-water cylinder | A separate cylinder stores hot water |
| When a tap opens | The boiler responds to hot-water demand | The tap draws from the stored hot water |
| Demand limits | Output and incoming water supply constrain delivery | Stored volume, recovery and supply constrain delivery |
A common mix-up
A cylinder does not prove the heating circuit is open vented, and a system boiler does not by itself establish whether the domestic hot-water cylinder is vented or unvented. Actual product designs vary. For gas work in Great Britain, use a Gas Safe registered engineer.
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Common mistakes
- Assuming a cylinder means the heating is open vented.
- Overlooking the need for a Gas Safe registered engineer for gas work in Great Britain.
Learning content, not a qualification: it won't make you competent, certified or registered. Follow the regulations that apply and a qualified plumber before acting. About our content
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