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An Air Admittance Valve is a small mechanical valve that is generally fitted on to the top of a 110mm soil pipe to allow air to be drawn into the drainage system. All above ground drainage systems should be vented, to prevent the build up of pressure in the system and to enable appliance traps (commonly referred to as U bends) to work correctly.
When a toilet is flushed, the movement of water creates a pressure inside the soil pipe. If the pipe is vented at the top of the run (i.e. open to the atmosphere), air is drawn into the system to balance this pressure. If however there is no vent and hence no air can be drawn in to the pipe, this pressure will create a siphoning effect. This effect in turn causes water to be drawn out of any nearby basin, sink or bath trap, allowing drain odours to enter into the room.
Normally the vent is an open end at the top of the pipe, usually protected by a vent terminal, which is a type of guard which prevents birds from entering. However, there are regulations in place to prevent pipes from terminating close to opening windows, including roof windows. This usually means that the pipe has to be high enough to finish above the level of the eaves - and on a system that is run inside the house this means that the pipe will have to penetrate the roof itself.
The best way to get around this problem is to install an air admittance valve (sometimes known as a Durgo valve). It is a cleverly designed valve which opens when internal pressure builds up, and draws air into the system. Crucially, when the valve opens no air is allowed to escape, meaning that all drainage gases and odours are contained within the drainage system.
These fittings are generally installed within loft spaces and inside ducts or cupboards. They need to be fitted above the 'spill over level', which is the highest point that water can reach in the drainage system. As the mechanism is very fine, the internal parts of the air admittance valve need to be protected from dust and frost. This is generally achieved by the fitting of a polystyrene cover which is supplied with the valve.
The very latest generation of air admittance valves includes a range that can be fitted externally. This range is available in three different colours and it allows external soil pipes to be terminated at a much lower level than traditional vent pipes that need to go above the eaves level, making the installation of the whole system a much safer job.
Air admittance valves are now a commonplace alternative to high level vent pipes, and many thousands are installed each year all over the world.