Air admittance valves in waste systems are used to prevent trap seals from being lost because of siphonage - an issue that is often indicated by drain odours in the room and a gurgling sound when any water is drained from the sink or basin. Because of this they are usually a retro-fit item.Air admittance valves are sometimes referred to as Durgo valves. They are cleverly engineered fittings that allow air to be drawn into the pipework to prevent siphonage when pressure builds up in the waste pipe, but crucially no foul odours can escape from the system.The valve is connected to the waste pipe using the integral compression coupling on the tail. This valve can be connected to most 40mm plastic waste systems and some copper waste pipes.It is usually installed in a waste tee that is cut into the main waste pipe run.It is manufactured to BS EN 12380:2002