Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A plumber is installing a main vent stack for a 5-story commercial building. The building's main soil or waste stack is 4 inches in diameter. What is the minimum required diameter for the main vent stack?
- A4 inches
- B2 1/2 inches
- C3 inches
- D2 inches
Show answer & explanationAnswer & explanation
Correct answer: C. 3 inches
According to most plumbing codes (e.g., IPC Table 906.1), the minimum diameter of a main vent stack is determined by the size of the building drain or the largest soil/waste stack served. For a 4-inch soil or waste stack, the minimum main vent stack diameter is 3 inches.
Why the other options are wrong
- A. While a 4-inch vent would be acceptable, it is not the minimum required size; codes often allow for a smaller vent than the stack it serves.
- B. A 2 1/2-inch vent is not a standard minimum size for this application.
- D. A 2-inch vent is typically too small for a 4-inch main soil/waste stack in a multi-story building.
Main Vent Stack Sizing
The determination of the minimum diameter for the main vent pipe that serves to ventilate the entire building drainage system, preventing siphonage and back pressure.
- Based on building drain or largest stack diameter.
- Also considers developed length and DFUs.
- Ensures adequate air circulation throughout DWV.
Memory trick: Vent size follows drain, but not in vain.