Trap
In plumbing
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.
Using Trap in a plumbing system
Updated
A distinction worth remembering
Pressure describes the driving force available; flow describes the amount delivered in a period. A restriction can reduce flow even when the static pressure is adequate. First identify which system feeds an outlet before comparing its behaviour with another.
Questions to take further
- Are the heating circuit and drinking-water supply the same water?
- They are different systems. A heating circuit circulates water for heat transfer, while the wholesome-water supply serves drinking and washing outlets. Connections between systems need appropriate protection against contamination.
Sources
- Skills England: plumbing and domestic heating technician — The systems, materials, planning and customer responsibilities in the England occupational standard.
- Water Regs UK: backflow protection — Contamination risks and why protection must match the fluid category and conditions.
Where Trap fits in the learning path
The Plumbing Basics section covers these topics:
- Water in a Home
Read, practise, return
1. Read it in context
- Water in a HomePlumbing Basics
2. Try recalling, then explore practice
Use the memory prompt below to connect the vocabulary.
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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The main vertical pipe that carries waste from toilets and other appliances down to the underground drain. Its top usually continues up into open air, or ends in an air admittance valve, so that air can get in and the water in the traps is not sucked out as waste rushes down.
Soil stack
Read the Soil stack entryCompare related terms
Open a meaning to compare its role with Trap, or follow its full entry.
Soil stack — Soil and vent pipe
The main vertical pipe that carries waste from toilets and other appliances down to the underground drain. Its top usually continues up into open air, or ends in an air admittance valve, so that air can get in and the water in the traps is not sucked out as waste rushes down.
Read the Soil stack 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.