How many fixture units does a 4" vent pipe have?

For a 4" vent pipe, the correct answer indicating that it has 240 fixture units is based on the principles outlined in plumbing codes, which define the capacity of vent piping in relation to the number of fixture units it can handle.

Fixture units are a measure of the probable demand on the drainage system, and they translate the flow of water use into a standardized measure for plumbing design. The 4" vent pipe is designed to accommodate a significant number of fixture units because of its larger diameter, which allows for better airflow and the management of drain pressures that can occur during heavy usage, thereby preventing siphoning and maintaining proper drainage system function.

This significant capacity is reflected in the chosen answer, acknowledging that proper venting is critical for maintaining the efficiency of the plumbing system and preventing issues like pressure imbalances that can lead to drainage problems. Understanding the relationship between pipe diameter, venting requirements, and fixture units is essential for effective plumbing design and compliance with applicable codes.