T&P relief valve
In plumbing · also called Temperature and pressure relief valve
A safety valve on an unvented cylinder that opens if the stored water gets too hot or the pressure too high, letting water out through the tundish and the discharge pipe. It is a last line of protection behind the cylinder's thermostats and cut-outs, and it is never capped, adjusted or tampered with.
Using T&P relief valve 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 T&P relief valve 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
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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 entryTundish
A funnel with a visible air gap on the discharge pipe from an unvented cylinder's safety valves. It lets anyone see when hot water is discharging, and it keeps that discharge apart from the drinking supply. Water running through a tundish means a safety device has operated, and the system needs checking by a competent person.
Read the Tundish 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.
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