Vented cylinder
In plumbing
A hot-water cylinder fed from a cold water storage cistern above it, with an open vent pipe that rises over that cistern. Its pressure comes from the height of the cistern, so outlets upstairs can run gently, and it does not carry the layered safety devices an unvented cylinder depends on.
Using Vented cylinder in a plumbing system
Updated
A distinction worth remembering
A cylinder fed from a cistern behaves differently from one sealed and fed from the main. Pressure, the parts on the inlet and the safety devices all follow from that one fact. Learn to tell the two apart and to see which faults belong to a competent unvented installer.
Questions to take further
- Is a dripping discharge pipe on an unvented cylinder a minor nuisance?
- No. It shows a relief valve is opening, so something in the system is not working as designed. The discharge route stays open and the fault goes to a competent installer.
Sources
- GOV.UK: Approved Document G — Guidance for England on hot-water safety, including stored hot water; guidance is not itself the legislation.
- Skills England: plumbing and domestic heating technician — The systems, materials, planning and customer responsibilities in the England occupational standard.
Where Vented cylinder fits in the learning path
The Hot Water Systems section covers these topics:
- Hot Water Cylinders
- Combis and Instant Hot Water
- Hot Water Temperature Control
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- Hot Water CylindersHot Water Systems
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A hot-water cylinder fed straight from the mains with no open vent or cold storage cistern, so it stores hot water under pressure. It depends on safety devices, including temperature and pressure relief, and on discharge pipework that carries hot water away safely if they operate. Recognise one and hand it over: in England, installing an unvented hot-water system that holds more than 15 litres is notifiable building work, done by a competent installer registered with a competent-person scheme or with building control told before work starts. Wales, Scotland and Northern Ireland have their own building rules.
Unvented cylinder
Read the Unvented cylinder entryCompare related terms
Open a meaning to compare its role with Vented cylinder, or follow its full entry.
Unvented cylinder
A hot-water cylinder fed straight from the mains with no open vent or cold storage cistern, so it stores hot water under pressure. It depends on safety devices, including temperature and pressure relief, and on discharge pipework that carries hot water away safely if they operate. Recognise one and hand it over: in England, installing an unvented hot-water system that holds more than 15 litres is notifiable building work, done by a competent installer registered with a competent-person scheme or with building control told before work starts. Wales, Scotland and Northern Ireland have their own building rules.
Read the Unvented cylinder entryCistern — Cold water storage cistern
An open-topped container that holds water at normal air pressure, such as the cold water storage cistern in a loft or the cistern behind a toilet. It is filled through a float-operated valve, and its lid, warning pipe and insulation help keep the stored water clean, stop it overflowing and protect it from frost.
Read the Cistern entryIndirect cylinder
A hot-water cylinder heated by water from a boiler or heat pump flowing through a coil inside it. The heating water and the water drawn at the taps never mix. A direct cylinder, by contrast, heats its own stored water, for example with an immersion heater.
Read the Indirect cylinder entryBefore you start: some plumbing work is regulated or notifiable
Some water fittings work needs notice to, and consent from, the water supplier before it starts. Check the rules where you live.
Gas work is for competent workers covered by a Gas Safe registered business in Great Britain. Northern Ireland has separate gas regulations. Do not work on gas pipes or appliances yourself.
In England, installing an unvented hot-water system over 15 litres is notifiable building work: use a competent installer registered with a competent-person scheme, or tell building control before work starts. Leave this work to a suitably skilled person.
Wales, Scotland and Northern Ireland have their own water and building rules; check which apply before work starts. Electrical connections belong with an electrician.
This guide does not qualify you or make you competent, approved or registered for plumbing, gas or unvented work.
A definition is a start. Using it in a real question is where it sticks.