air admittance valve
An air admittance valve is a large valve fitted to the top of ventilating pipes which terminate internally within the building. It is designed to allow fresh air into the pipe but prevents odors from escaping.
Air admittance valves should not be used on discharge stacks connecting to a drain which has an intercepting trap or to drains which are subject to surcharging as it could result in the trap seal loss of appliances. There should only be a limited number of dwellings within the local vicinity fitted with these valves thus ensuring a good air circulation within the drain.