3.1 Vent Stacks, Relief Vents & Vent Sizing Tables

The performance rule behind every vent table

SPS 382.31 (3) (a) states the goal: every trap and trapped fixture gets an individual vent unless the chapter allows another method. Vent systems must be designed so that a trap seal "shall be subject to a maximum pneumatic pressure differential equal to one inch of water column." Every table in this chapter exists to meet that 1-inch limit.

Stacks that must be vented

Vent stack connections. The vent stack connects to the drain stack at or below the lowest branch drain connection, using a wye-pattern fitting in a vertical part of the stack. The vent stack and stack vent then either run to a vent terminal, join a vent stack that does, or connect to a stack vent at least 6 inches above the flood level rim of the highest fixture on the drain stack. Where a vent stack joins another vent, the joint must be at least 38 inches above the next higher floor where vented fixtures are located, and never less than 2 inches above the highest flood level rim served.

Relief vents for tall stacks. On stacks over 10 intervals, the relief vent's lower end connects with a wye below the horizontal branch on that floor, and its upper end ties to the vent stack at least 38 inches above the next higher floor with fixtures on the stack.

Building drain relief vents. The relief vent connects with a wye within 2 feet upstream of the top of the change in elevation. Its connection to another vent is at least 38 inches above the next higher floor. It must be at least half the diameter of the building drain.

Vent grade and connection heights (SPS 382.31 (15))

1. All vents must be graded to drain back to a drain pipe by gravity.
2. Except for wet vents, a vent connects to horizontal drain piping above the horizontal centerline of the drain.
3. A vent serving a wall-outlet fixture may not run horizontally lower than 36 inches above the floor, and never below the highest flood level rim it serves (return vents and combination systems excepted).
4. Vent piping may not connect to a branch vent lower than 38 inches above the floor, and never less than 2 inches above the highest flood level rim served.

Sizing stack vents and vent stacks (Table 382.31-2)

Stack vents and vent stacks are sized from the diameter of the drain stack at its base and the developed length of the vent. The developed length of a stack vent is measured from the highest drain branch connection to the vent terminal or header. For a vent stack, it is measured from its base connection to the terminal or header.

A 1-1/2 inch stack vent may serve no more than 2 water closets or similar flush-action fixtures of 4 or more dfu. Yoke vents are sized as vent stacks.

Sizing individual, common, branch and circuit vents and vent headers (Table 382.31-3)

These vents are sized by the dfu they serve and the developed length from the farthest fixture drain served to the point where the vent joins a larger vent or the terminal.

Footnotes: a 1-1/4 inch vent may serve no water closets, and a 1-1/2 inch vent may serve no more than 2 water closets or similar fixtures of 4 or more dfu. A vent header uses the sum of the dfu of all stacks it vents and may not be smaller than any vent connecting to it.

Worked example 1. A branch vent serves 20 dfu with a developed length of 120 feet. Use the 24-dfu row: 1-1/2 inch allows only 50 feet, 2-inch allows 150 feet, so the vent is 2 inches.

Worked example 2. A 4-inch drain stack needs a stack vent with a 100-foot developed length. In Table 382.31-2 a 2-inch vent is limited to 20 feet and a 3-inch vent to 180 feet, so use 3 inches.

Relief vent sizing. A relief vent for a circuit-vented branch must be at least half the branch diameter. A relief vent for a horizontal wet vent must be at least 1-1/2 inches. Lengths for both come from Table 382.31-3.