Journeyman Plumber ExamDrainage, Waste, and Venting (DWV) SystemsHard
A plumber is sizing a vent pipe for a 3-inch horizontal branch drain that receives discharge from multiple fixtures with a total drainage fixture unit (DFU) load of 25. The vent is connected within 5 feet of the trap arm of the most distant fixture. What is the minimum diameter of the vent pipe required?
- A2 inches
- B1 1/2 inches
- C4 inches
- D3 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 2 inches
According to IPC Table 906.1 (or similar code tables), for a 3-inch horizontal branch drain with a DFU load of 25, the minimum size of the vent pipe is determined by the total DFU load and the developed length of the vent. For 25 DFUs, a 2-inch vent is typically required. While a 3-inch drain can carry more DFUs, the vent sizing is based on the DFU load it serves, not just the drain size.
Why the other options are wrong
- B. 1 1/2-inch vent is too small for a 25 DFU load, regardless of the drain size.
- C. This size is excessively large and not the minimum required for the given DFU load.
- D. While a 3-inch vent would work, it's not the minimum required size for 25 DFUs; this size is typically for much higher DFU loads.
Vent Pipe Sizing (DFU)
Vent pipe sizing is based on the total drainage fixture unit (DFU) load served by the vent, the developed length of the vent, and the size of the drainage pipe it serves. It ensures adequate air circulation and prevents trap siphonage.
- Based on DFU load.
- Based on vent developed length.
- Minimum 1/2 size of drain served.
- Critical for DWV system function.
Memory trick: Vent Size Depends on Load and Length, Giving the System Its Strength.