Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsMedium
A plumber is installing a battery of three lavatories, each with a 1 1/2-inch trap, connected to a horizontal branch drain. The branch drain is 15 feet long. What is the minimum required size for the vent pipe serving this branch?
- A1 1/4 inches
- B3 inches
- C2 inches
- D1 1/2 inches
Show answer & explanationAnswer & explanation
Correct answer: D. 1 1/2 inches
According to plumbing codes, the minimum size of a vent pipe for a branch drain is generally half the diameter of the drain pipe it serves, but not less than 1 1/4 inches. For a 1 1/2-inch branch, a 1 1/2-inch vent is required.
Why the other options are wrong
- A. 1 1/4 inches is the absolute minimum for any vent, but not sufficient for a 1 1/2-inch or larger branch.
- B. 3 inches is excessively large for venting this setup.
- C. 2 inches would be acceptable but is larger than the minimum required.
Branch Vent Sizing (Minimum)
The smallest allowable diameter for a vent pipe serving a horizontal branch drain, determined by the size of the drain it protects.
- Vent size must be at least half the drain size.
- Absolute minimum vent size is typically 1 1/4 inches.
- Ensures proper air circulation and prevents trap siphoning.
Memory trick: Vent's Verdict: Half the Drain, Never Less.