How many fixture units does a 2" vent pipe contain?

The correct answer is based on the guidelines of plumbing codes that define the capacity of pipe sizes to handle the drainage load of plumbing fixtures. A 2-inch vent pipe is allocated a specific number of fixture units, which are used to measure the potential flow load that the pipe can accommodate without causing hydraulic issues or excessive back pressure in the plumbing system.

In this case, a 2-inch vent pipe is designated as having a capacity of 24 fixture units. This standard reflects the size's ability to handle the airflow necessary for venting while supporting the drainage from associated plumbing fixtures. Vent pipes play a crucial role in maintaining proper pressure in the drainage system, allowing for efficient operation and preventing the siphoning of traps.

The other choices do not align with the established codes regarding the venting capacities of different pipe sizes. The recognized standards provide a basis for engineering safe and effective plumbing systems, ensuring that each component is properly sized for its intended use.