To understand the purpose and function of a Plumbing Drainage Venting system, one must first understand the key components of the traditional plumbing system:

- The plumbing fixture (i.e. the waste outlet) from a sink / W.C./ basin

- A water trap seal (the U-Bend you see attached to your sink fixture)

- A waste pipe feeding the waste into the municipal wastewater drainage system

- Air Admittance Valves or vent pipes through the roof of the building, connected in parallel to the waste pipe, which allows air to enter and exit the waste pipe.

Vent pipes or Air Admittance Valves are at the heart of the plumbing drainage venting system, and are a crucial component: They allow the air pressure within the waste pipe to equalise with the ambient (external) air pressure. This is very important, since a difference in air pressure between waste pipe and ambient would result in one of two problems:

- If the air pressure inside the waste pipe were lower than the ambient air pressure (“negative air pressure”), this would suck the water out of the trap seal. This problem is sometimes referred to as “induced siphonage”. The purpose of the trap seal is to provide a barrier between the waste pipe and the fixture in order to prevent odorous and sometimes dangerous sewer gases to enter the building. Negative pressure occurs each time a fixture is operated and water exits through the waste pipe.

- If the air pressure inside the waste pipe becomes suddenly higher than the ambient air pressure, this excess pressure will also compromise the trap seal. In many cases, a sudden increase in pressure (sometimes referred to as “positive transient”) will even cause the wastewater contained within the fixture to spray the room. The hygiene and health risks of positive transients are considerable, particularly in areas with bacterial diseases such as SARS. Tall buildings are particularly susceptible to this problem if the issue is not adequately addressed.

Planning and implementing an effective plumbing drainage venting system is therefore a crucial consideration in the design and construction of any building. Air Admittance Valves offer many advantages to designers and architects over a traditional vent-pipe based systems, since complexities and costs can be reduced.