An Air Admittance Valve (AAV) is a practical solution used to regulate air pressure within a soil pipe system, preventing negative pressure from causing problems like siphoning and drain odours. It is an essential component, especially in places where traditional vent pipes are impractical or unsightly.
What is an Air Admittance Valve?
An Air Admittance Valve (AAV) is a one-way valve that allows air to enter a soil pipe when negative pressure builds up due to waste water flow (e.g., when a WC is flushed). This negative pressure, if not relieved, can siphon water out of traps in basins, baths, and sinks, leading to unpleasant odours entering the home.
Traditionally, vent pipes were used to allow air to enter the drainage system, which balanced the pressure. However, these vent pipes needed to terminate high above the roof or eaves, which could be unsightly and subject to strict regulations. The AAV addresses this by offering a more discreet solution while preventing siphoning.
How Do Air Admittance Valves Work?
Negative Pressure Relief: When the waste water flows through the soil pipe, it creates a siphoning effect, resulting in negative pressure. The AAV opens automatically to allow air into the system, breaking the siphon and balancing the pressure.
One-Way Function: The key feature of an AAV is that it is a one-way valve. This means that while air can enter the soil pipe to regulate pressure, foul air cannot escape back into the room. The clean air enters the pipe, but no sewer gases are allowed to leak out.
Where Should an Air Admittance Valve Be Installed?
Height Requirement: The AAV must be installed at least 200mm above the highest water entry point in the system. This ensures that it effectively prevents siphoning without interfering with normal drainage.
Installation Location: While AAVs can be installed inside, they are often placed in lofts, ducts, or cupboards for aesthetic reasons. This allows for easier installation and avoids visible pipework.
Can Air Admittance Valves Be Used Outside?
Yes, some AAVs can be used outside. For example, the Floplast AX110 External Air Admittance Valve is designed for outdoor use. It features frost protection and insect protection, making it suitable for external installations, particularly when replacing an external soil stack. This external model eliminates the need to route the soil pipe above the eaves, which is typically required for traditional vent pipes.
How Do Air Admittance Valves Fit to the Soil Pipe?
External AAVs: The external AAVs, such as the Floplast AX110, typically feature rubber fins that provide a push-fit joint into the soil pipe, making installation quick and easy. This type of connection is similar to the joint used on a WC pan connector.
Internal AAVs: For internal installations, the valve is typically solvent welded to the soil pipe. Push-fit versions may also be available, offering flexibility in installation methods.
Summary
An Air Admittance Valve is a useful device for regulating air pressure in a soil pipe system. By opening when negative pressure builds up, it prevents siphoning and the entry of foul odours into the home. AAVs offer an efficient alternative to traditional vent pipes, particularly in installations where external venting is impractical. Whether installed inside or outside, AAVs provide an effective and aesthetically pleasing solution to maintain a healthy and odour-free drainage system.
