Vented vs unvented hot-water cylinders
Supports revision for: C&G 6035 Unit 206. Coverage references only; no awarding-body affiliation or qualification is conferred.
Vented and unvented describe how a stored hot-water system deals with pressure and expansion. A typical vented cylinder is supplied from a cold-water cistern with an open vent. An unvented cylinder has a sealed hot-water store supplied under mains pressure through its control arrangement.
Plumbing learning · Sources checked 29 October 2026
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0Where does a vented hot-water cylinder get its cold water from?
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| Question | Vented | Unvented |
|---|---|---|
| Where does supply pressure come from? | Usually the height of the cold-water storage cistern | The incoming mains, subject to the system’s controls and available supply |
| Where does expansion go? | The open-vented arrangement accommodates expansion | A designed expansion space or vessel accommodates expansion |
| What protects the store? | Temperature controls and a clear vent arrangement matter | Temperature controls, pressure controls and safety discharge arrangements work together |
A common mix-up
Unvented does not mean unlimited flow or maintenance-free operation. The available supply and the designed safety controls both matter. Direct/indirect is a separate distinction about the heat source. Unvented-cylinder installation and safety-device work need a competent specialist; this comparison is not a procedure.
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Key numbers
- 15 litres
- In England, an unvented cylinder bigger than this goes through a competent-person scheme installer or building control first.
Common mistakes
- Thinking unvented means unlimited flow.
- Treating 'indirect' as another word for 'unvented'.
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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