Unvented systems, understood
Stored Hot Water in Depth · Plumbing
An unvented cylinder stores hot water at mains pressure, so it carries its own safety parts. This topic is about recognising them, understanding what each one does and knowing when to hand the job over.
The cold feed carries the inlet group and an expansion vessel, though some cylinders trap a bubble of air at their top instead. On the cylinder are a control thermostat, a non-self-resetting energy cut-out and a temperature and pressure relief valve. If the thermostat fails, the cut-out stops the heat; if that fails too, the relief valve lets water out before it can boil.
Water grows by about 1.7 per cent when it is heated from 10 °C to 60 °C, and a sealed cylinder needs room for it. Any discharge runs along D1 to a tundish with a visible air gap, then down D2, which drops straight first and then falls all the way to a safe, visible end.
In England, a new store holding over 15 litres counts as notifiable building work: a scheme-registered installer notifies it, or building control is told before work starts. Wales, Northern Ireland and Scotland have their own rules. Installing and servicing one is for a skilled installer, so recognise the parts and hand the work over.
Reviewed
Recognise the safety parts, then hand the job over
An unvented cylinder holds hot water at mains pressure, so it has a thermostat, an energy cut-out, a temperature and pressure relief valve, and a discharge pipe that has to end somewhere visible and safe. Learn to spot each one, and leave installing, servicing and faults to a skilled installer.
Questions to take further
- Is Approved Document G the law on unvented cylinders?
- No. In England and Wales the law is the Building Regulations 2010, whose requirement G3 covers hot water storage; Approved Document G is the guidance on meeting it. In England, putting in a cylinder of more than 15 litres is notifiable: a scheme-registered installer notifies it, or building control is told before work starts. Wales, Scotland and Northern Ireland have their own rules.
Sources
- GOV.UK: Approved Document G — Guidance for England on hot-water safety, including stored hot water; guidance is not itself the legislation.
Key numbers
- 15 litres
- In England, putting in a hot water storage system bigger than this is notifiable building work.
Common mistakes
- Boxing in a tundish so a discharge goes unseen.
- Running D2 uphill, or ending it where someone could be scalded.
- Adjusting a safety valve instead of handing the fault over.
Lessons on the plumbing path
- The safety chain on an unvented cylinder
- Expansion: vessels and air gaps
- Discharge pipework: tundish, D1 and D2
- Service, notification and paperwork
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.