The sizing of all elements of vent piping design is governed by specifications and regulations. This includes the size of stack vents and vent stacks, individual, branch, circuit, and relief vents, as well as multiple branch and ejector vents.
The minimum diameter of vent stacks and stack vents is worked out from the “developed length” of the pipeline and the total number of drainage fixtures that are connected to them. There is also an important rule of thumb, which is that the diameter may never be less than half the diameter of the drain that the stack vent or vent stack is serving, or less than 1½ inches (38 mm) in diameter.
The Code provides an easy-to-read table to determine sizes. This shows the diameter of soil or waste stacks in inches, the total number of fixture units to be vented, and the maximum developed length of vent in feet (measured from the vent connection to the open air), and diameter of the vent in inches.
The developed length of any individual, branch, circuit, or relief vent is measured from the farthest point of the vent connection to the drainage system, to the point that it connects to the stock vent or vent stack or termination outside of the building.
The diameter of individual and other vents needs to be at least half the required diameter of the drain served, but vent pipes must be at least 1½ inches (38 mm) in diameter. Vents are sized in accordance with the table mentioned above unless there are more than 10 branch intervals, in which case a similar but different table is used.
There are also tables to determine the size of ejector vents including sewage pumps that may or may not be pneumatic. Those that are not pneumatic operate at atmospheric pressure and receive drainage discharge under gravity flow conditions.