Illinois State Plumbing Practice Exam 2026 - Free Plumbing Practice Questions and Study Guide

Vent sizing ensures air can enter the drainage system to prevent the toilet’s trap from being siphoned and to allow smooth drainage. A single water closet has enough discharge that the code requires a vent large enough to handle that load, and the smallest vent permitted for it is 2 inches. Using any smaller vent would restrict air entry and increase the risk of trap siphon and sewer gas escaping. A vent larger than 2 inches is permitted if needed for other fixtures or longer vent runs, but the minimum for one toilet remains 2 inches. So the minimum vent size for a single water closet is 2 inches.