Tundish
In plumbing
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.
Using Tundish 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 Tundish 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
- Hot Water SafetyIsolation, Hot Water Safety and Simple Repairs
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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 Tundish, 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 entryT&P relief valve — 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.
Read the T&P relief valve entryAir gap
A physical gap between the outlet of a pipe or tap and the highest level the water below it could reach, so that water cannot be drawn back into the supply. Because there is nothing in it that can fail, an air gap is the protection used against the highest-risk water, such as a category 5 source. A tap whose spout sits well above the spill-over level of a basin is a familiar example.
Read the Air gap 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.