Trap seal
In plumbing · also called Water seal
The water left standing in a trap after an appliance empties. It blocks foul air coming up from the drain. Its depth is measured from the dip of the trap up to the outlet weir, and in England the drainage guidance expects part of it to survive normal use and testing.
Using Trap seal in a plumbing system
Updated
A distinction worth remembering
Work out what each appliance discharges, which trap suits its route and how deep a seal it starts with. Then ask what could pull that seal out: its own flow, another appliance's flow, or a long spell with nobody using it.
Questions to take further
- Why does my sink smell of drains?
- Often the trap has lost its water, through siphonage, evaporation from an outlet nobody uses, or a slow leak. Running the tap refills it; if the smell keeps coming back, the venting or the pipe layout may need looking at.
Sources
- GOV.UK: Approved Document H — Guidance for England on above-ground drainage, traps, venting, access and rainwater disposal; guidance is not itself the legislation.
Where Trap seal fits in the learning path
The Above-Ground Sanitation and Drainage section covers these topics:
- Sanitary Appliances and Traps
- Waste and Soil Pipework
- Blockages, Drains and Sewers
Read, practise, return
1. Read it in context
- Sanitary Appliances and TrapsAbove-Ground Sanitation and Drainage
2. Try recalling, then explore practice
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3. Return to the meaning
After reading or practising, explain the idea in your own words, then check what you remembered.
Recall a related term
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A bend or pot under a sink, bath, shower or toilet that always holds some water. That water, called the trap seal, stops foul air from the drains coming back into the room. A trap that has dried out, been siphoned empty or been left off lets drain smells into the building.
Compare related terms
Open a meaning to compare its role with Trap seal, or follow its full entry.
Trap
A bend or pot under a sink, bath, shower or toilet that always holds some water. That water, called the trap seal, stops foul air from the drains coming back into the room. A trap that has dried out, been siphoned empty or been left off lets drain smells into the building.
Read the Trap entrySiphonage
The loss of water from a trap when moving water lowers the air pressure behind its seal. Self-siphonage comes from the appliance's own discharge running full bore; induced siphonage comes from another appliance's flow passing through a pipe the trap shares.
Read the Siphonage 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.