Vents and Traps
Vents and Traps. This domain tests trap seals, venting, and the air path that protects drainage. Expect questions on trap seal depth, trap arms, vent stacks, stack vents, wet venting, air admittance valves, and the reasons traps lose their seal.
Core rules
- Every fixture trap must be vented, except where the code expressly permits common venting or a special arrangement. The trap seal is normally 2 to 4 in deep.
- A trap arm connects the trap to its vent. It must be within the code length limit for its size and must not fall so much that the vent opening is blocked by waste.
- Vents must extend to open air, be sized by fixture units, and terminate at the required height above the roof and away from openings. AAVs are limited by code and require access and listing.
- Wet venting is allowed only where the code permits; it has rules for fixture types, pipe sizes, and vertical-to-horizontal relationships.
Worked example
A 2 in trap arm runs 7 ft at ¼ in per ft. The fall is 7 × 0.25 = 1.75 in. That fall is less than a typical trap seal depth, so the arm will not drain the trap by slope alone. But the installation is still acceptable only if 7 ft is within the code maximum trap-arm length for 2 in and the vent is correctly installed. The reasoning: trap-arm acceptability requires both length and slope checks.
Common traps
Do not assume a trap is vented by the drain alone. Do not use an AAV where a code-required vent to open air is mandatory. Do not confuse a stack vent with a vent stack. Do not let a trap arm exceed its length limit.