Hot water cylinder
Plain English. Practical detail. UK context.
A hot water cylinder stores heated water for domestic use. It may be vented or unvented and may be heated directly by an immersion heater, indirectly through a heat exchanger, or by a combination of sources. The cylinder's usable service depends on its system design, not simply the volume printed on its label.
Last reviewed: (editorial review against the public sources listed below).
Identify the system before comparing replacements
A vented arrangement generally works with an open vent and a storage supply arrangement, whereas an unvented system is sealed and has dedicated safety provisions. These are not interchangeable installation details. A thermal store also has a different role from a cylinder storing the water delivered to taps. Homeowners should describe existing equipment and the problem, rather than order a replacement on height and diameter alone. Access for bringing the unit in and replacing it later also matters.
Matching demand and heat source
The installer considers household use, simultaneous outlets, recovery performance, incoming water conditions and the heat source. A heat-pump-compatible cylinder needs a suitable design; attaching a new label does not change the heat exchanger. A larger store may not improve a shower if the supply flow is the limiting factor. Controls, insulation and distribution losses also affect practical operation. Quotations should state the complete proposed system, including any electrical or pipework changes, instead of comparing vessel prices as though every installation were equivalent.
Safety and national scope
Approved Document G provides England's guidance on hot-water supply and safety, including the safety arrangements relevant to unvented storage. Water Regs UK explains the need to accommodate expansion and prevent overheating and overpressurisation. Other UK nations have their own building standards provisions. A competent installer must design the controls and discharge arrangement as a system. Do not cap a relief pipe, remove a safety valve or attempt intrusive repair because a cylinder is dripping or making unusual sounds.
Living with and maintaining the installation
At handover, ask how to use the normal controls, what servicing is required and what visible discharge or fault indications mean. Keep the model, commissioning record and instructions. Storage temperature, scald protection and hygiene must be considered together rather than adjusting every control to the same number. Tradespeople should leave safety discharge routes visible where intended and access available for maintenance. An unexplained pressure loss, repeated safety discharge or overheating needs diagnosis; merely changing the most accessible valve can miss a failed control or expansion problem elsewhere.
Frequently asked questions
Is an unvented cylinder always better?
No. Suitability depends on the supply, demand, heat source, building and installation constraints. Compare designed systems rather than labels.
Does a bigger cylinder guarantee stronger showers?
No. Pressure, available flow, pipework and the shower specification can be limiting factors regardless of storage volume.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.