Why must the vent pipe opening never be below the weir of the fixture trap?

The reason the vent pipe opening must never be below the weir of the fixture trap is rooted in the need to maintain the trap seal, which is crucial for preventing sewage gases from entering the living space. When the vent pipe is positioned below the weir of the fixture trap, it creates a scenario where negative pressure or siphoning can occur in the drain line. This negative pressure can lead to the self-siphoning of the trap seal, effectively lowering or entirely draining the water barrier that protects the interior environment from harmful gases.

By ensuring that the vent pipe opening is located above the trap's weir, the system allows for proper air circulation, which helps to equalize pressure in the drainage system. This prevents the siphoning effect that could compromise the trap seal and allows the fixture to function as designed, ensuring safety and sanitation in the plumbing system.