# What is an air admittance valve?
**An air admittance valve (AAV) is a one-way mechanical vent fitted to a drainage system that allows air to enter the pipework when negative pressure occurs, preventing the siphoning of water from traps.** It is also known as a Durgo valve or an aerator and serves as an alternative to a traditional open vent pipe through the roof.
## How does an air admittance valve work?
When water flows through a drainage pipe, it creates negative pressure (a partial vacuum) behind it. Without ventilation, this suction can pull the water seal out of nearby traps, allowing foul sewer gases to enter the building. The AAV solves this by:
– **Opening under negative pressure** — a diaphragm or sealing membrane lifts to admit air into the pipe
– **Closing under positive pressure or at rest** — gravity and the membrane seal the valve shut, preventing odours and gases from escaping
– **Maintaining the water seal** — by equalising pressure, the trap retains its water barrier
## Application
Air admittance valves are used in:
– **Kitchen and bathroom waste pipes** — where extending a vent stack to the roof is impractical or costly
– **Renovation projects** — adding drainage points without altering the existing vent system
– **Island sinks** — where no wall is available to run a conventional vent pipe
– **Loft conversions** — where routing a new vent pipe through the roof is undesirable
### Installation guidelines
– Mount the AAV vertically, at least 200 mm above the highest water entry point in the branch
– Ensure the valve remains accessible for inspection and replacement
– An AAV must not replace the main vent stack entirely; at least one open vent to atmosphere is required per building in most codes
– Use only valves that comply with EN 12380 or the relevant national standard
### Limitations
– AAVs only admit air; they do not release positive pressure, so they cannot replace a full vent system on their own
– They contain a moving part that can stick or degrade over time, requiring periodic inspection
– Not all building codes permit their use in every situation
## Related terms
– Water trap
– Vent stack
– Drainage system
– Siphoning
– Soil pipe
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