Illinois State Plumbing Practice Exam

Vents are sized to prevent trap siphonage and to let air into the drainage system as wastewater flows out. A toilet discharges a large and rapid volume of water, which can create a strong vacuum in the trap if the vent is too small. The code sets a minimum vent size for a single water closet at 2 inches to ensure enough air can enter the system and keep the trap seals from being siphoned. That’s why the correct choice is 2 inches. Smaller diameters like 1/2 inch or 3/4 inch would not provide adequate venting, while 3 inches is larger than necessary for a single fixture unless there are special conditions.