The most important purpose of venting traps is to

• A. To prevent backflow into fixtures
• B. To minimize sewer gas entry
• C. To keep vacuum or pressure on traps to a minimum
• D. To relieve odors

Answer: (C)

The main idea is that venting traps prevents siphonage by balancing air pressure in the drainage system. When a fixture drains, the moving water can create a vacuum behind it. Without an air path, that vacuum can pull the water out of the trap, breaking the water seal that blocks sewer gases from entering the building. The vent provides a ready air supply so pressure stays balanced and the trap seal remains intact. That maintenance of the trap’s water seal is what keeps sewer gases from entering fixtures.

Backflow into fixtures is controlled by proper piping design and backflow prevention devices, not by venting. Minimizing sewer gas entry isn’t the vent’s function by itself—the trap seal does that, and the vent’s role is to prevent loss of that seal. Odor relief can occur if a trap dries out or fails, but the most important purpose of venting is to keep the trap from being siphoned and losing its seal.