What Is an Unvented Cylinder?
Written by James Walker · Last reviewed 16 August 2026
James Walker has 2 degrees in construction (BSc (Hons) Building Surveying · MSc Environmental Design of Buildings) and an extensive background in issue resolution, resolving over £4 billion of consumer issues.
An unvented cylinder is a sealed hot water storage tank fed directly from the mains, so it delivers hot water at mains pressure. It replaces the older arrangement of a tank in the loft feeding a cylinder below.
Why people fit them
Mains-pressure hot water at every outlet, and no cold water tank in the loft, which frees the space and removes a common source of leaks.
They suit houses with more than one bathroom better than a combi boiler does, because they can supply several outlets at once.
The qualification point
Installing or servicing an unvented cylinder requires a specific qualification. A plumber is not automatically qualified for it, and many are not.
This is a safety matter, not paperwork. A sealed vessel holding hot water at mains pressure has to have working safety devices, and the installation is notifiable under the Building Regulations.
Ask directly whether the person is qualified for unvented systems, and check it.
Annual servicing
They need servicing annually, the expansion vessel and safety valves in particular. Skipping it can invalidate the warranty and, more importantly, allows a safety device to fail unnoticed.
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