Last updated on Thu, 02/10/2011 - 18:49

A drainage pipe is normally at neutral air pressure compared to the surrounding atmosphere. When a column of waste water flows through a pipe, it compresses air in the pipe, creating a positive pressure that must be released or it will push back on the waste stream and downstream traps' water seals. As the column of water passes, air must flow in behind the waste stream or negative pressure (suction) results. The extent of these pressure fluctuations is determined by the fluid volume of the waste discharge.

The purpose of ventilation stacks is to control pressure in the pipework in order to prevent foul air from the waste water system entering the building. You will find below some major configurations principles, but combinations and variations are often required. See standard EN 12056-2.
Black water and grey water can be drained either in separate or in a single discharge stack. According to the selected option, the rules for duct sizing are different.

Primary ventilated system configurations

Control of pressure is achieved by air flow in the discharge stacks. The soil stacks extend in main roof vents to above and out of the roof.

Alternatively, air admittance valves may be used. They are pressure-activated, one-way mechanical vents, used in a plumbing system to eliminate the need for conventional pipe venting and roof penetrations.

A single stack is still possible but design precautions have to be taken to prevent self-siphonage.

Secondary ventilated system configurations

In buildings of three or more storeys - if the air pressure within the drain becomes suddenly higher than ambient, this positive transient could cause waste water to be pushed into the fixture, breaking the trap seal.

Vent stacks are put in parallel to waste stacks to allow proper venting and prevent such disorders. Air admittance valves may also be used, in this configuration.

Under many building codes, a vent stack, a pipe leading to the main roof vent, is required to the draining fixtures (sink, toilet, shower stall, etc.).

To allow only one vent stack, and thus one roof protrusion as permitted by local building code, sub-vents may be tied together and exit a common vent stack.