Contains a diaphragm which under light spring pressure is normally held closed, containing foul air within the soil system. When there is a discharge from an appliance, a negative air pressure is produced in the system, causing the diaphragm in the valve to lift from its seat, allowing air to be drawn into the system. This intake and the rapid equalisation of air pressure prevents foul air escaping and prevents the water seals in traps from being broken.
An air admittance valve must be installed in a vertical position above the flood level of the highest appliance connected to any soil stack. It should be situated in a non-habitable area of the building, e.g. the roof space, where it will be easily accessible and there is reduced risk of freezing.
The underground drain or branch drain which serves a stack or stacks to which air admittance valves are fitted may require additional ventilation at a position further up stream from the stack connection. This will minimise the effects of excessive back pressure if a blockage should occur in a drain.