Venting Systems and Traps
Venting Systems and Traps
This domain tests trap seal protection and vent design. Expect questions on trap arm length, vent terminal height, wet venting, air admittance valves, and the difference between a vent and a drain.
Core rules
- Trap seal depth is 2 to 4 in. The trap arm must be vented within the distance allowed for its size: commonly 5 ft for 1-1/4 in, 6 ft for 1-1/2 in, 8 ft for 2 in, 12 ft for 3 in, and 16 ft for 4 in.
- A vent terminal must extend above the roof per code and be located away from windows, doors, and air intakes; a common rule is at least 6 in above the roof, with greater height near walls.
- Wet vents and common vents must be sized and arranged so that one fixture cannot siphon another.
- Air admittance valves are allowed only where the code permits; they are not a universal substitute for a vent stack through the roof.
- A crown vent, an S-trap, and a flat or bellying vent are prohibited because they promote seal loss.
Worked example
A 2 in lavatory drain is 7 ft from the stack. The allowed trap arm for 2 in is 8 ft, so the arrangement is acceptable if the pipe slopes uniformly and the vent rises vertically. If the run were 10 ft, the trap arm would exceed the limit and the fixture would need a separate vent or relocation closer to the stack.
Traps that cost marks
Forgetting that trap arm length is measured from the trap weir to the vent, installing an AAV in a concealed location, connecting a vent to a drain below the fixture, and assuming a larger drain automatically allows a longer trap arm.