Air Admittance Valve

These are NOT normally referred to as AAV's

Big ones (which fit 100mm diameter pipe or above) are often known by the trade-name Durgo. They are designed to let air IN to waste or drain system, so that siphoning does not occur. Such siphoning can “pull” the water out of a trap so smelly air passes upwards. They do NOT let air out. Individual fittings are available from 32mm diameter up, or they can be built in to a trap, such as this one - the little part which looks tacked on the top is the Air Admittance Valve. The traps are known as Vented or Anti-Siphon traps.

Courtesy of ChrisR