3.3 Return Vents, Combination Drain & Vent Systems & Air Admittance Valves

Return vents for islands and enclosures (SPS 382.31 (12))

A kitchen island sink has no wall to carry a vent up. Wisconsin's answer is the return vent:

1. Extend an individual vent, vertical wet vent or common vent as high as possible under the fixture enclosure, then return it vertically downward and connect it to the fixture drain or branch drain with a wye-pattern fitting.
2. Horizontal vent piping connects to the vertical section of the fixture vent and runs to a point where it can rise to a vent terminal or join another vent.
3. All vent piping below the floor must use drainage fittings and slope at least 1/4 inch per foot to the drainage point, because it can fill with water during a stoppage.
4. Provide cleanouts on the vent piping under SPS 382.35.

Combination drain and vent systems (SPS 382.31 (17))

A combination drain and vent system uses oversized drain piping to carry both water and air. Wisconsin permits four types.

Stacks

A second stack option serves one kitchen sink above one wall-outlet fixture of 2 dfu or less in a dwelling unit, with no other fixtures. The stack is at least 2 inches below the sink and 4 inches below the lower fixture, or 3 inches for its entire length below the sink. It may not offset above the lower fixture's connection.

Building drains and subdrains

A building drain may serve as a combination drain and vent for floor drains and floor-outlet fixtures if:

• A vent or drain of at least 2 inches connects upstream of any building drain branch.
• No more than 2 water closets connect through building drain branches.
• The building drain between the branch and the upstream vent is one pipe size larger than Table 382.30-3 requires, and at least 3 inches. The vent is at least half that diameter and at least 2 inches.
• Floor drain and floor-outlet fixture traps (other than water closets) are at least 3 inches.
• No branch connects within 20 pipe diameters downstream of a stack base fitting.
• Branch pitch and length meet Table 382.31-1.

Island laboratory sinks and elevator threshold drains

Island laboratory sinks may use a horizontal combination drain one size larger than Table 382.30-2 requires. It needs a 2-inch minimum vent or drain stack upstream, 3-inch fixture drains, individual connections, and a looped vent under each sink with a cleanout. Elevator threshold drain stacks are a minimum of 6 inches with 4-inch traps, no offsets, and a vent at least 3 inches (and half the largest stack diameter) rising from above the highest branch.

Air admittance valves (SPS 382.31 (17m))

An air admittance valve (AAV) opens to admit air on negative pressure and closes by gravity. Wisconsin treats it only as a termination point. Amendments to (17m) took effect September 1, 2026 under CR 26-019.

Where an AAV may serve. It may terminate a branch vent, circuit vent, common vent, individual vent, wet vent or combination drain and vent system. It may serve a pumped-discharge clothes washer standpipe only when the fixture drain downstream of the vent point is at least 3 inches. A branch with AAV-vented fixtures that connects to a stack with 4 or more branch intervals above it needs a relief vent between the most downstream fixture and the stack.

Where an AAV may not be the vent termination (selected items from the 18-item list):

• Negative-pressure areas, laboratories and commercial parking structures
• Vents that relieve positive pressure, and stack vents serving 2 or more branch intervals
• Vent stacks required by SPS 382.31 (4) (a), and vents serving sumps
• Chemical waste fixtures, POWTS holding or treatment tanks, and FOG treatment devices
• Supply or return air plenums, enclosed stairwells, and pits or vaults below grade
• Clean rooms, health care facilities, licensed restaurant kitchens, residential bedrooms and daycares

Sizing. Table 382.31-6 limits the developed length from the fixture to the AAV:

Installation. The AAV must be:

1. Vertical, within plus or minus 15 degrees of plumb.
2. At least 4 inches above the top of the horizontal pipe it serves.
3. No more than 20 inches below the flood rim of any fixture it serves.
4. At least 6 inches above insulation.
5. Accessible, in a space that admits air.
6. Backed by at least one 3-inch or larger vent on the same building drain that extends outdoors.

The vent system an AAV serves may offset horizontally less than 36 inches above the floor if it doesn't connect to another vent.

Testing and documentation. Test the AAV before or after installation at 1 inch of water column. If the pressure falls 0.5 inch or less in 1 minute, it passes. The contractor must give the owner a copy of the manufacturer's written AAV description.