8.3 Wet Venting and Circuit Venting (UPC 908, 911; WA 908.2.4 and 911.1)

What Wet Venting Is

A wet vent is a vent that also serves as a drain. It works because small fixtures discharge little water, so an oversized drain still has room for air to move above the flow. Section 310.4 generally prohibits using a waste pipe as a vent, but 908 through 911 are the listed exceptions. Washington's amendment to 310.4 refers to them the same way.

Vertical Wet Venting (UPC 908.1)

• Limited to vertical drainage piping receiving the trap arms of one- and two-fixture-unit fixtures, where that pipe also serves as the vent for not more than four fixtures.
• All wet-vented fixtures must be within the same story. A fixture with a continuous vent discharging into the wet vent must also be on that story.
• No wet vent may exceed 6 feet of developed length.
• 908.1.1 Size: The vertical pipe between two consecutive inlet levels is a wet-vented section. Each section must be one pipe size larger than the minimum waste size of the upper fixture, or one size larger than the minimum size for the total DFU it serves, whichever is larger. It may never be less than 2 inches.
• 908.1.2 Vent connection: The common vent is sized for the total fixture units served. It may not be smaller than the minimum vent for any fixture served, or than 904 requires.

Example: A lavatory (1 DFU, 1-1/4-inch minimum) above a bathtub (2 DFU, 1-1/2-inch minimum) on the same story. The lavatory's drain down to the tub inlet is a wet-vented section. One size above 1-1/4 inches is 1-1/2 inches, but the 2-inch floor controls, so the section is 2 inches. The dry vent above the lavatory is sized for the 3 DFU it serves. A 1-1/4-inch vent carries only 1 DFU, so the dry vent must be at least 1-1/2 inches (8 DFU).

Horizontal Wet Venting for a Bathroom Group (UPC 908.2)

A bathroom group on the same floor level may be vented by a horizontal wet vent when all of the following conditions are met:

Example (private): A lavatory (1 DFU) is individually vented, and its horizontal drain serves as the wet vent for a bathtub (2) and a water closet (3) that connect downstream of it. DFU discharging into the wet vent = 1 + 2 + 3 = 6, which is 5 or more, so the wet vent must be 3 inches. If the water closet were connected downstream of the wet-vented section, leaving only the lavatory and tub (3 DFU) on it, 2 inches would suffice.

Circuit Venting (UPC 911 as Amended by Washington)

Circuit venting lets a battery of fixtures on one horizontal branch share a single circuit vent. The horizontal branch itself acts as the vent for the fixtures along it.

Washington 911.1 Circuit Vent Permitted:

1. A maximum of eight fixtures on a horizontal branch drain may be circuit vented.
2. Each fixture drain connects horizontally to the branch.
3. The branch is classified as a vent from the most downstream fixture drain connection to the most upstream fixture drain connection.
4. Restaurant kitchen equipment may not be connected to a circuit-vented system, because of its grease-producing potential.
5. Each trap arm connects horizontally to the branch within the Table 1002.2 limits.
6. Exception: Back-outlet and wall-hung water closets may be circuit vented if no floor-outlet fixtures connect to the same branch.

Other circuit venting rules:

• 911.1.1: Circuit-vented branches may be connected together. Each group of up to eight fixtures is a separate circuit vent.
• 911.2: The circuit vent is at least 2 inches. It connects between the two most upstream fixture drains and rises vertically from the horizontal branch.
• 911.3: The vent section of the branch slopes no more than 1 inch per foot, and its entire length is sized for the total drainage discharge to the branch. Interconnected branches are each sized independently, and the downstream branch carries the total.
• 911.4 Relief vent: A 2-inch relief vent is required when the circuit-vented branch receives four or more water closets and connects to a drainage stack that receives soil or waste from upper branches. It connects between the stack and the most downstream fixture drain, on the vertical rising from the branch. It may be a fixture drain for a fixture in the same branch interval, but its discharge may not exceed 4 fixture units (911.4.1 and 911.4.2).
• 911.5: Other fixtures on the same floor may discharge to the branch if they are individually or common vented.

Example: Six floor-outlet water closets on a 4-inch horizontal branch on the third floor of a five-story building. Up to eight fixtures are allowed. The circuit vent is 2 inches, connected between closets 5 and 6 (the two most upstream). Because there are four or more water closets and the stack receives upper floors, a 2-inch relief vent is added between the stack and closet 1.