A positive air pressure reduction device for sanitary drainage systems is a device specifically designed to protect water trap seals against unwanted positive pressure transients in sanitary drainage systems. Positive pressure reduction devices are to be used in building drainage waste and vent (DWV) systems. They are intended to reduce the impact of short duration air pressure transients that arise in DWV networks through use. They are not intended to have any effect on long duration or steady-state offsets in air pressure.

The device consists of a variable volume reservoir contained within a ventilated rigid outer casing with an inlet connection by which the reservoir inflates when subjected to positive pressure. In its inactive state, the flexible reservoir is deflated. Expansion only occurs in response to an increase in line pressure at the entrance to the device. This expansion provides a variable volume reservoir for air. The device connects to the drainage network via an airtight seal to prevent the diversion of airflow from entering the reservoir. As a result, the reservoir becomes an integral part of the drainage network.