A plumber is installing a 3-inch horizontal branch drain that will serve three water closets and two lavatories. The water closets each have a drainage fixture unit (DFU) value of 4, and the lavatories each have a DFU value of 1. According to common plumbing codes, what is the minimum required size for the vent pipe connected to this branch drain?
- A3 inches
- B2 inches
- C1 1/2 inches
- D1 1/4 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 2 inches
The total DFU for the branch drain is 3 water closets * 4 DFU/closet + 2 lavatories * 1 DFU/lavatory = 12 + 2 = 14 DFU. For a 3-inch branch drain, the minimum vent pipe size is determined by the total DFU and the length of the vent. However, a common rule is that the vent pipe should be at least one-half the diameter of the drain pipe it serves, but never less than 1 1/4 inches. Here, half of 3 inches is 1 1/2 inches. But, for a 3-inch horizontal branch drain serving fixtures with a total DFU of 14, a 2-inch vent is required to adequately prevent trap siphonage and back pressure, considering typical code tables for vent sizing.
Why the other options are wrong
- A. 3 inches is the same size as the drain, which is usually only required for vent stacks or very long vents serving high DFU loads.
- C. 1 1/2 inches would be half the drain size, but specific DFU tables for 3-inch drains often require a larger vent for this load.
- D. 1 1/4 inches is generally the minimum size for any vent, but insufficient for this DFU load on a 3-inch drain.
Branch Drain Vent Sizing
The process of determining the minimum diameter of a vent pipe required for a horizontal branch drain to ensure proper air circulation and prevent trap seal loss.
- Vent size depends on the total DFU served by the branch drain.
- Vent size depends on the diameter of the branch drain.
- Vent size depends on the developed length of the vent pipe.
- Most codes require the vent to be at least half the diameter of the drain, but never less than 1 1/4 inches.
Memory trick: Don't Forget Vents, Diameter, and Developed Length!