Venting Systems: Vent Types, Sizing, and Installation
Venting Systems: Vent Types, Sizing, and Installation
Venting questions test why air matters: without a vent, a trap loses its seal by siphonage or back pressure. Expect questions on vent types, sizing, terminal locations, and the rules that keep a vent from becoming a drain.
Types and where they apply
- Individual vent: serves one trap.
- Common vent: two traps on the same floor on one vertical vent.
- Wet vent: a drain that also vents another fixture - allowed only within the code's conditions (typically a bathroom group, with size and fixture limits).
- Circuit vent: vents a row of fixtures on one horizontal branch, with relief venting as the run grows.
- Stack vent / vent stack: the stack vent extends a drainage stack above the roof; a vent stack serves lower floors separately.
- Island (loop) vent: for sinks at islands where no wall vent is possible.
- Air admittance valves: only where the code allows, listed, accessible, and never a substitute for the required open vent to outdoors.
Rules that get tested
Each trap needs a vent; the vent must rise vertically at least 6 in. above the fixture's flood-level rim before any offset so it cannot act as a drain. Vent size follows the DFU load and drain size from the code table. Vents slope back toward the drain so condensate returns; no sags, no crown vents. Terminate outdoors, unobstructed, above the roof, and clear of windows, doors and air intakes at code distances.
Worked example. A second-floor lavatory vent offsets horizontally just above the trap. Because that offset sits below the flood-level rim, waste can foul it and the fixture can siphon. The fix is to keep the vent vertical until 6 in. above the rim, then offset - or relocate the vent into the wall.
Traps. Wet-venting a kitchen sink; sizing a vent by its own fixture instead of the DFU total; using an AAV in place of a missing roof vent; running a vent with a rise then a fall that traps water.