What is a Air Admittance Valve

An Air Admittance Valve (AAV), also commonly referred to as a Durgo valve, is a specialist plumbing component used in soil and waste systems to regulate air flow. Its primary function is to admit air into a drainage system when negative pressure occurs, such as when water is discharged through a pipe. The valve opens momentarily to equalise pressure, preventing siphonage of water traps, and then closes automatically to maintain a sealed system. When closed, it blocks the escape of foul odours from the drainage system into the building.

Air Admittance Valves have become a popular alternative to traditional open soil vent pipes, particularly in situations where external venting is impractical or aesthetically undesirable. They are widely used in residential and commercial properties, especially in loft conversions, basement installations, and retrofitting projects.

How Air Admittance Valves Work

In a typical plumbing system, negative pressure is created when water flows rapidly through vertical pipes, such as when a toilet is flushed or a bath is emptied. This negative pressure can pull water out of nearby traps, leading to the release of foul gases from the drainage system. Traditionally, vent pipes terminating above roof level would allow air in to counter this effect.

Air Admittance Valves perform the same function internally. The valve remains sealed under normal conditions, preventing the escape of air or gas. When negative pressure is detected, a spring-loaded or gravity-operated mechanism opens the valve briefly, allowing air to enter the pipe and equalise the pressure. Once equilibrium is restored, the valve reseals.

This mechanism allows for greater flexibility in the design and layout of plumbing systems, especially where venting through the roof is not possible.

Advantages and Applications

Air Admittance Valves offer several practical and economic benefits, making them a preferred solution in many installations. These include:

  • Elimination of roof penetration: Ideal for buildings where roof access is difficult or where external vent pipes would disrupt the appearance of the structure. 
  • Space-saving design: Compact and easy to install, particularly in tight spaces such as behind walls or in cupboards. 
  • Reduced material and labour costs: Fewer pipe runs and roof work required compared to traditional venting systems. 
  • Improved retrofit compatibility: Suitable for upgrading older properties where installing new vent pipes would be disruptive or costly. 

Common applications include:

  • Single fixture or group venting in bathrooms or kitchens 
  • Loft conversions and basement bathrooms 
  • Internal installations where external venting is not feasible 
  • Systems with long branch pipes or complex layouts 

Regulations and Installation Considerations

In the UK, the use of Air Admittance Valves is permitted under Building Regulations, specifically Approved Document H. However, certain conditions must be met to ensure safe and effective operation. These include:

  1. Location – AAVs must be installed at least 200mm above the highest trap they serve and must be accessible for maintenance. 
  2. Ventilation limits – They may not be suitable as the sole means of ventilation for extensive drainage systems; at least one open vent to the atmosphere is usually still required. 
  3. Use in sealed spaces – The surrounding area must have adequate ventilation, and valves must not be installed in completely airtight enclosures. 

Only valves that comply with recognised standards, such as BS EN 12380, should be used, and professional installation is strongly recommended to avoid drainage issues.

Conclusion

Air Admittance Valves provide a modern, efficient solution to air pressure regulation in plumbing systems. By admitting air only when needed and sealing at all other times, they prevent trap siphonage and odour release without the need for extensive vent pipework. When properly selected and installed, they offer a reliable and discreet alternative to traditional soil vent pipes, contributing to both functional performance and design flexibility in contemporary drainage systems.