3.2 Common Vents, Wet Vents & Circuit Vents

Why these methods exist

An individual vent on every trap is the default (SPS 382.31 (3) (a)), but it's expensive in walls and ceilings. Wisconsin permits common vents, wet vents, circuit vents, return vents, combination drain and vent systems and limited air admittance valves as alternatives. Each is allowed only inside its own fence of conditions. Exam questions usually test one condition: a fixture count, a minimum diameter or a dfu limit.

Common vents (SPS 382.31 (11))

Vertical drains. A common vent may serve a maximum of 2 fixtures when both fixture drains connect to a vertical drain at the same elevation.

• If the connection is a sanitary tee with a side inlet, the centerline of the side inlet may not be below the centerline of the larger opening.
• A double sanitary tee may not be used for a blowout-type fixture, a kitchen sink with a food waste grinder or pumped-discharge appliance, or a clothes washer served by a common vent. Their discharge can blow across into the opposite drain.

Horizontal branches. The drains from 2 wall-outlet fixtures, each 1 dfu or less, or from the 2 traps of a kitchen sink (with or without a dishwasher), may join a horizontal branch without individual vents if a common vent connects downstream of both fixture drains. Both fixture drains must be the same diameter, and the developed length from the vent to the farthest trap must meet Table 382.31-1.

Vertical wet vents (SPS 382.31 (13) (a))

When 2 wall-outlet fixtures on the same floor connect to the same vertical drain at different elevations, the lower fixture drain may be wet vented by the drain of the upper fixture if:

1. No other fixture discharges into the vertical drain above or between the two (fixtures may connect below).
2. A branch vent connects immediately above the higher fixture drain connection.
3. The drain between the 2 fixtures is at least one pipe size larger than the upper fixture drain and not smaller than 2 inches.
4. Both fixture drains meet the trap-to-vent distances of Table 382.31-1.
5. The higher fixture is not a water closet.

Horizontal wet vents for bathroom groups (SPS 382.31 (13) (b))

This is the classic house-bathroom arrangement. The drain from a lavatory or lavatories, which are individually or common vented, may serve as the wet vent for not more than 2 bathtubs or showers and not more than 2 water closets:

Where required, the relief vent connects downstream of the water closet drains and upstream of any other fixture drain, and it must be at least 1-1/2 inches.

Worked check. Two lavatories (1 dfu each) discharge into the 2-inch wet vent, for 2 dfu, which is within the limit. Four lavatories would reach the 4-dfu maximum for a 2-inch wet vent, and a fifth would require a larger wet vent. Adding a kitchen sink or clothes washer to the lavatory drain is prohibited outright, because no other fixtures may discharge into or be served by the wet vent.

Other wet vents (SPS 382.31 (13) (c))

An individual vent serving a floor-outlet fixture, a common vent for floor-outlet fixtures, a circuit vent, or a relief vent for a circuit-vented or wet-vented drain may itself act as a wet vent if:

• No more than 2 wall-outlet fixtures, each 1 dfu or less, drain into it.
• The wet vent is at least 2 inches.
• The fixtures draining into it are on the same floor as the fixtures it vents.
• The branch vent it joins is sized under SPS 382.31 (14).

Circuit venting (SPS 382.31 (10))

A circuit vent may serve a horizontal drain with at least 2 but not more than 8 wall-outlet fixtures, or 2 to 8 floor-outlet fixtures, on the same branch. Blowout-type fixtures and wall-outlet carrier water closets are excluded.

1. The circuit vent connects to the horizontal drain between the 2 most upstream fixtures.
2. A circuit-vented drain with 4 or more fixtures needs a relief vent downstream of the most downstream circuit-vented fixture and upstream of any other drain connection.
3. Two circuit-vented branches serving 8 fixtures (4 on each) need at least one relief vent, unless they drain to a stack with no connections above. One relief vent can serve both if it is downstream of where they join.
4. The circuit-vented drain may not diminish in size from the most downstream fixture drain (or from the relief vent) to the circuit vent connection.
5. Each fixture drain must meet the trap-to-vent limits of Table 382.31-1, measured to its connection with the circuit-vented branch.
6. Extra wall-outlet fixtures of 1 dfu or less may discharge into the branch if they have their own individual or common vents.

The relief vent for a circuit-vented branch must be at least half the branch diameter. The circuit vent itself is sized from Table 382.31-3.

Worked example. Six floor-outlet water closets in a row on a 4-inch branch: the circuit vent connects between closets 1 and 2 (the most upstream). Because there are 4 or more fixtures, a relief vent of at least 2 inches connects just downstream of closet 6. Six public closets carry 36 dfu, so with a 60-foot developed length the circuit vent from the 42-dfu row of Table 382.31-3 is at least 2 inches (2-inch limit 100 feet). A 1-1/2 inch vent would fail both the 30-foot length and the 2-water-closet limit.