Air admittance valve
In plumbing · also called AAV
A one-way valve on top of a stack or branch that opens to let air in when flowing water lowers the pressure inside, then closes so drain smells stay in. It cannot let air out, so a drainage system still needs open ventilation somewhere, normally an open stack.
Using Air admittance valve in a plumbing system
Updated
A distinction worth remembering
Check the fall of each pipe, where each branch meets the stack, and how air reaches the pipe behind the water. Most gurgling, slow or smelly wastes trace back to one of those three.
Questions to take further
- Can an air admittance valve replace a vent pipe?
- It can stand in for an open vent on some stacks and branches inside a building, where the guidance for that nation allows it. A valve only lets air in, so the drainage system still needs an open vent somewhere to let air out.
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 Air admittance valve 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
- Waste and Soil PipeworkAbove-Ground Sanitation and Drainage
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
Which term fits this definition? Say it to yourself, then reveal it. This is a memory prompt, with no score or saved result.
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 Air admittance valve, 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 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.