Venting Systems and Vent Sizing
Why vents exist
A vent equalizes pressure so the trap seal survives. Without it, flow past a trap siphons the seal and sewer gas enters. Every trap must be vented, the vent must connect above the trap weir, and it must rise above the fixture's flood level rim before any horizontal offset. The vent must terminate outdoors above the roof, clear of windows and air intakes.
Types
Individual, common, wet, circuit and stack vents; relief vents for stacks; combination waste-and-vent systems in limited applications; air admittance valves only where the code permits them. Wet venting lets one pipe serve as drain and vent under strict limits on fixture types, sizes and length. Vent sizing comes from the vent table keyed to DFU and developed length - undersized vents cause slow drains and gurgling, not just noise.
Traps
- Venting a trap below the weir or below the flood level rim, which turns the vent into a drain.
- Using an AAV as a substitute where the code requires a vent through the roof.
- Wet-venting a water closet through a lavatory drain without meeting size and length limits.
- Forgetting that the vent terminal must be located and sized for snow, wind, and separation from other terminals.
- Ignoring the vent's developed length when sizing.
Worked example
A lavatory with a 1-1/4 in. trap arm may run only a limited distance to its vent. If the fixture is moved farther from the stack, the trap arm exceeds the table and the fix is usually to re-vent, not to upsize the drain. Upsizing the drain does not cure a vent problem.
Rule of thumb: the vent protects the seal, not the flow. Size it from the vent table, not from the drain size.