Glossary · Central heating, radiators and hot water
What is an unvented cylinder?
An unvented cylinder is a hot water storage vessel fed with cold water straight from the mains supply pipe, with no vent pipe and no loft tank. Because the water is sealed in under pressure, it must carry safety devices that stop the stored water going above 100°C. It is also called a mains pressure hot water cylinder.
Key points
- It is fed from the mains, so hot water reaches taps and showers at mains pressure without a pump.
- The building regulations require safety devices that stop the stored water exceeding 100°C.
- Discharge pipework from the safety devices must be visible and must carry hot water safely to a place open to the air.
- Installing one is notifiable building work, unless the installer is registered with a competent person scheme.
- Choose an installer who can show a competent person scheme registration or a current operative skills card for unvented systems.
How an unvented cylinder works
An unvented cylinder is fed with cold water from the supply pipe, not from a storage cistern in the loft. Approved Document G, the building regulations guidance for hot water safety in England, defines it as a vessel with no vent pipe, in which water is heated directly or indirectly.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments) The water expands as it heats. In an unvented system that expansion is taken up either inside the cylinder or in a separate expansion vessel.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments)
The main benefit is pressure. BASI describes an unvented cylinder as one that takes mains pressure cold water directly, heats it and delivers it at full pressure to every tap and shower. That is why showers can feel different from a loft-tank system, without a pump.[2]BASI Heating, Hot water cylinders If your mains supply is weak, the cylinder cannot improve it, so a pressure check comes first. See low water pressure.
The safety devices and rules
Sealed hot water under pressure needs protection. The regulations say a hot water system with a storage vessel must stop the stored water ever going above 100°C, and must convey any discharge from the safety devices safely to where it is visible and causes no danger.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments)
| Requirement | What the guidance says |
|---|---|
| Overheating protection | At least two independent safety devices, such as those that release pressure, in addition to any thermostat |
| Discharge pipework | Visible at some point and carried safely to a place open to the atmosphere |
| Temperature relief | Devices such as a pressure relief valve discharge through a short metal pipe to a tundish |
| Notification | Notifiable building work, unless the installer is in a competent person scheme |
Who can fit one
This is not work for a householder or a general handyman. Approved Document G says the safety and performance of an unvented system depends on the right choice of system, the right safety devices and correct installation. It tells owners to pick installers with the necessary skills, shown by registration with a competent person scheme or a current registered operative skills card for unvented hot water systems.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments)
The installation is notifiable building work. The Building Control body must be told before work starts, unless the installer is registered with a competent person scheme. A registered installer self-certifies the work and the owner receives a building regulations certificate.[1]Ministry of Housing, Communities and Local Government, Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments) BASI states that its engineers who quote cylinder jobs are G3 qualified and that a commissioning sheet is left with the customer.[2]BASI Heating, Hot water cylinders
Example
Priya lives in a 1930s semi in Bradford and has a loft tank cylinder. Her shower is weak and she is replacing her boiler. Her installer checks her incoming mains supply, then quotes an unvented cylinder as an upgrade. The installer is registered to fit them, so the work is self-certified and Priya receives a building regulations certificate. BASI lists hot water cylinder work from £995 at a fixed price.[2]BASI Heating, Hot water cylinders
What people get wrong
Many people assume an unvented cylinder boosts a weak supply. It does not. It delivers whatever pressure the mains provides, so a poor supply stays poor.
The second mistake is treating it as an ordinary plumbing job. The discharge pipe, the safety devices and the paperwork are part of the installation, not extras. An unvented cylinder with a blocked or altered discharge route is a safety risk, so leave all adjustment to a registered installer.
The bottom line
An unvented cylinder stores mains-fed hot water under pressure. That gives strong flow from every outlet, as long as the supply itself is good. The price of that benefit is a set of safety devices, a safe discharge route and a qualified installer. Ask to see the registration before you agree a job.
Common questions
What is mains pressure hot water?
It is hot water delivered at the pressure of the incoming mains supply. An unvented cylinder is fed straight from the mains, so hot and cold taps and showers run at similar pressure.
Can any heating engineer fit an unvented cylinder?
No. Approved Document G says owners should choose installers with the skills to do this work, shown by a competent person scheme registration or a current operative skills card for unvented hot water systems.
Does fitting an unvented cylinder need Building Control involvement?
Yes, in one of two ways. The work is notifiable to Building Control before it starts. An installer registered with a competent person scheme self-certifies instead and you receive a building regulations certificate.
Why does an unvented cylinder have a discharge pipe?
The safety devices release water if the pressure or temperature gets too high. The regulations require that discharge to be visible and to run safely to a place open to the air.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Ministry of Housing, Communities and Local Government. Sanitation, hot water safety and water efficiency: Approved Document G (2024 amendments). Checked 11 November 2026. ↩
- BASI Heating. Hot water cylinders. Checked 11 November 2026. ↩