Hot Water Cylinders
Hot Water Systems · Plumbing
A cylinder keeps a store of hot water ready to use. This topic explains the vented and unvented kinds, how each is heated, and where the legal line sits for unvented work.
A vented cylinder is filled from a cold cistern above it and has an open vent, so its pressure depends on the height of that cistern. Hot water rises to the top of the store, which is why the draw-off leaves from the top and cold water enters at the bottom.
A direct cylinder is heated by an element inside it. An indirect cylinder is heated by boiler water passing through a coil, and the coil keeps the boiler's water apart from the water people wash in.
An unvented cylinder is sealed and fed from the main, with a chain of inlet controls and layered safety devices. In England and Wales, an unvented system storing over 15 litres counts as notifiable building work. One main route is a competent installer who belongs to a competent person scheme and notifies the job; the other is notice to building control before starting. Scotland and Northern Ireland set their own rules.
Know how the store is fed before judging it
Updated
How to reason about the system
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.
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Lessons on the plumbing path
- How a vented cylinder works
- Direct and indirect heating
- Inside an unvented system
- Why unvented work is restricted
Terms to know
Before 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.