Venting Systems: Protecting Trap Seals
Why venting matters
Venting equalizes pressure in the drainage system so trap seals are not siphoned or blown out. Every trap must be protected by a vent. Vent systems include individual vents, common vents, wet vents, circuit vents, and stack vents. Each has specific rules about sizing, length, and connection height.
Vent types and rules
An individual vent rises from the trap arm. A common vent serves two fixtures at the same level. A wet vent carries waste from one fixture while venting another, and it has strict limits on fixture types and distance. A circuit vent serves a battery of fixtures with a single vent. A stack vent extends the soil stack above the roof. Vent terminals must be located away from windows and air intakes.
Sizing and length
Vent sizing depends on fixture units and the developed length of the vent. Long vents may need to be increased in size. The vent must connect above the trap weir and below the fixture's flood level rim.
Traps in questions
Watch for vents that connect too low, vents that are too small, and vents that terminate too close to openings. Also watch for wet venting of fixtures that are not permitted to be wet vented.
Worked example
A lavatory and a water closet share a wet vent. The water closet must be the downstream fixture, and the lavatory must be within the allowed distance. If the water closet is upstream, the arrangement is not a permitted wet vent.
Rule of thumb
Every trap needs a vent; the vent must rise above the trap weir and terminate safely above the roof.