AIR ADMITTANCE VALVES
Air admittance valves are designed to decrease the number of external roof and wall surface penetrations required to accomodate soil and ventilating stacks without reducing the effectiveness or performance of the system. They also reduce the quantity of components required to complete a system.

The valve contains a diaphragm which under light spring pressure is normally held closed, containing foul air within the system. When there is a discharge from an appliance, a negative air pressure is produced in the system.This pressure causes 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.



Air admittance valves are packaged in formed polystyrene boxes, the tops of which should be fitted to the valve top after installation, providing insulation for the valve in use.

Air admittance valves can be fitted to sanitary pipe work systems of buildings up to ten storeys high. 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. In determining the requirement for additional ventilation to the underground drainage system the following rules can be used for general guidance.

1. Up to and including four domestic dwellings up to three storeys high, additional drain ventilation is not necessary.

2. Where an underground drain serves more than four such dwellings which have soil systems fitted with an air admittance valve, the drain must be vented as follows:

a. In the case of five to ten such dwellings, additional conventional ventilation must be provided at the head of the underground drainage system.
b. In the case of eleven to twenty such dwellings, additional conventional ventilation must be provided at the head and midpoint of the underground drainage system.

All multi-storey dwellings require additional venting of their underground drainage system if more than one such building, equipped with air admittance valves, is connected to a common drain not ventilated by conventional means.

These principles are illustrated in the diagrams: