What is the smallest vent size allowed for a 3-inch drain line?

What is the smallest vent size allowed for a 3-inch drain line?

• A. 1/2 inch
• B. 1 1/2 inches
• C. 2 inches
• D. 3/4 inch

Answer: (B)

Vent sizing depends on the drain size. The vent’s job is to admit air to the system as wastewater flows, preventing negative pressure from siphoning traps. For a 3-inch drain line, the smallest vent size permitted by typical plumbing-code tables is 1 1/2 inches. This size provides enough air passage to vent the line without causing trap siphonage under normal fixture loads. Vent sizes smaller than that would be undersized and risky, while a 2-inch vent, though acceptable, is simply larger than the minimum required.

Vent sizing depends on the drain size. The vent’s job is to admit air to the system as wastewater flows, preventing negative pressure from siphoning traps. For a 3-inch drain line, the smallest vent size permitted by typical plumbing-code tables is 1 1/2 inches. This size provides enough air passage to vent the line without causing trap siphonage under normal fixture loads. Vent sizes smaller than that would be undersized and risky, while a 2-inch vent, though acceptable, is simply larger than the minimum required.