6.3 Circuit Vents, Combination Waste & Vent, Island, Waste Stack & Single-Stack Venting

Why this section matters

Where individual dry vents are impractical, as in long restroom batteries, open floors and kitchen islands, the IPC allows special venting methods. Each has a fixture limit, a connection rule and a sizing table. Exam questions test those three things.

Waste stack vent (913)

• A waste stack may serve as the vent for all fixtures discharging to it if it is vertical, with no horizontal or vertical offsets between the lowest and highest fixture connections.
• Each fixture drain connects separately to the stack.
• The stack may not receive water closets or urinals (913.2).
• The stack vent is at least the size of the waste stack. Any offsets in it must be at least 6 inches above the highest flood level (913.3).
• Table 913.4 sizes the stack, which must be the same size for its whole length:

Circuit venting (914)

• 914.1: No more than eight fixtures connected to a horizontal branch drain may be circuit vented. Each fixture drain connects horizontally to the branch. The branch counts as a vent from the most downstream fixture connection to the most upstream one. Several circuit-vented branches may connect together, each group of up to eight counting separately (914.1.1).
• 914.2: The circuit vent connects between the two most upstream fixture drains. It follows Section 905, so it rises 6 inches above the flood rims, and it may not receive any soil or waste.
• 914.3: The vent section of the branch slopes no more than 1 unit vertical in 12 horizontal (8.3%) and is sized for the total drainage discharge to the branch over its full length. When branches connect together, the downstream branch is sized for everything it receives (914.3.1).
• 914.4, relief vent: Required where the circuit-vented branch receives four or more water closets and connects to a drainage stack receiving soil or waste from upper horizontal branches. It connects between the stack and the most downstream fixture drain (914.4.1). It may serve as a fixture drain for fixtures in the same branch interval, carrying no more than 4 dfu (914.4.2).
• 914.5: Other fixtures may discharge to the branch if they are on the same floor and are individually or common vented.
• Vent sizes follow 906.2: at least half the required branch size, and one size larger over 40 feet.

Worked example: restroom battery

Six public 1.6-gpf water closets (4 dfu each) sit on one horizontal branch that enters a 4-inch stack carrying three floors above.

1. Eligibility: six fixtures, each draining horizontally into the branch. That is no more than eight, so circuit venting is allowed.
2. Branch size: $6 \times 4 = 24$ dfu. Table 710.1(2) allows 20 dfu on a 3-inch branch and 160 on a 4-inch branch, so use 4 inches for the whole vent section.
3. Circuit vent: connect between closets 1 and 2 (the most upstream pair). Minimum size is half of 4 inches, which is 2 inches.
4. Relief vent: there are four or more water closets and the stack receives upper branches, so a relief vent is required between closet 6 and the stack. Use 2 inches.

The loop vent described in older texts works the same way, except the vent loops back to the stack vent instead of a vent stack. Section 905.5 still requires loop vents to be at least 6 inches above the highest flood rim.

Combination waste and vent system (915)

• 915.1: May serve only floor drains, sinks, lavatories and drinking fountains. It may never serve a clinical sink, water closet or urinal.
• 915.2: The only vertical pipe in the system is the connection from a fixture drain to the horizontal combination pipe, and it may be no more than 8 feet tall.
• 915.2.1: Slope no more than 1/2 inch per foot (4%), and no less than Table 704.1.
• 915.2.2: Size by Table 915.2.2. The horizontal length is unlimited.

• 915.2.3: Provide a dry vent connected at any point in the system, or connect the system to a horizontal drain that serves vented fixtures on the same floor. A system connecting to a building drain that receives only stack discharge must have a dry vent. The vent rises 6 inches above the highest flood rim before offsetting.
• 915.2.4–915.2.5: The vent is sized for the total dfu per 906.2. Each fixture drain connects within the Table 909.1 distance, and the combination pipe serves as that fixture's vent.

Island fixture venting (916)

• 916.1: Island venting is allowed only for sinks and lavatories. A residential kitchen sink with a dishwasher, a disposer or both qualifies.
• 916.2: The island vent connects to the fixture drain like an individual or common vent. It must rise vertically above the drainage outlet of the fixture before offsetting horizontally or turning down. Where several island fixtures share a branch vent, that vent must reach at least 6 inches above the highest island fixture before connecting to the outdoor terminal.
• 916.3: The vent piping below the flood rim is installed as drainage piping under Chapter 7, with drainage fittings and slope, but is sized as a vent (906.2). Its lowest point connects full size to the drainage system, either to a vertical drain or to the top half of a horizontal drain. Cleanouts must allow rodding of all vent piping below the flood rim, and rodding in both directions through one cleanout is allowed.

Typical layout:

1. The sink trap's fixture drain enters a fitting.
2. The vent rises inside the cabinet above the trap outlet, then returns down through the floor using drainage fittings.
3. Below the floor, the vent's low point ties full size into the drain.
4. A branch vent continues from there to a wall, then rises to at least 6 inches above the island rim before joining the vent system.
5. Cleanouts are placed so every below-rim vent section can be rodded.

Single-stack vent system (917)

A drainage stack sized under Table 917.2 may serve as its own vent. Key limits:

• The stack vent is the same size as the stack. A 3-inch stack may serve no more than two water closets (917.2).
• No more than one water closet may discharge into a 3-inch branch within 18 inches of the stack (917.3).
• Water closet connections may be no more than 4 feet from the stack, or 8 feet with a sanitary tee. Other fixtures may be no more than 12 feet (917.4).
• The vertical portion of a fixture drain must be 2 inches (3 inches for urinals and standpipes), with a drop of no more than 4 feet (917.5).
• No fixture connections within 2 feet above or below a horizontal offset (917.7).
• Stacks taller than two branch intervals may not receive branches from the lower two floors (917.8).

Exam traps

• Circuit vent location: between the two most upstream fixtures.
• Relief vent trigger: four or more water closets plus a stack that receives discharge from upper branches.
• Combination waste and vent: never water closets, urinals or clinical sinks. Maximum slope 1/2 inch per foot.
• The "2 feet from an offset" ban belongs to the single-stack system in 917.7.