7.4 Circuit Venting, Combination Waste & Vent, Island, Single-Stack & Air Admittance Valves

Circuit venting (IPC 914)

A circuit vent connects to a horizontal drainage branch and vents two traps up to eight traps or trapped fixtures connected in a battery (IPC Chapter 2).

• 914.1: no more than eight fixtures on a horizontal branch may be circuit vented. Each fixture drain must connect horizontally to the branch. The branch counts as a vent from the most downstream fixture drain connection to the most upstream one. The 2021 IPC does not limit the method to particular fixture types.
• 914.1.1: circuit-vented branches may be connected together. Each group of up to eight fixtures is treated as a separate circuit.
• 914.2: the circuit vent connects between the two most upstream fixture drains. It connects to the horizontal branch per Section 905 (above the centerline, then rising 6 inches above the flood level rim before turning horizontal). The circuit vent pipe must not receive any soil or waste.
• 914.3: the slope of the vent section of the horizontal branch may be no greater than 1 unit vertical in 12 units horizontal (8.3%). The entire vent section is sized for the total drainage discharge to the branch.
• 914.3.1: where circuit-vented branches are interconnected, each is sized independently, and the downstream branch is sized for its own fixtures plus everything upstream.

Relief vents on circuit-vented branches (914.4)

A relief vent is required for circuit-vented horizontal branches that receive the discharge of four or more water closets AND connect to a drainage stack that receives soil or waste from upper horizontal branches. Both conditions must be met.

• 914.4.1: the relief vent connects to the horizontal branch between the stack and the most downstream fixture drain of the circuit, and is installed per 905.
• 914.4.2: the relief vent may be a fixture drain or fixture branch for fixtures in the same branch interval as the circuit-vented branch, with a maximum discharge to the relief vent of 4 dfu.
• Relief, circuit and branch vents are sized by 906.2: at least one-half the required drain diameter and at least 1-1/4 inches.

914.5: other fixtures on the same floor may discharge to the horizontal branch if they are individually or common vented.

Combination waste and vent systems (IPC 915)

A combination waste and vent system uses a horizontal waste pipe that is larger than normal, so air moves freely above the flow line. It vents the fixtures connected to it (IPC Chapter 2).

• 915.1: it may serve only floor drains, sinks, lavatories and drinking fountains. It may not receive water closets, urinals or clinical sinks.
• 915.2: the only vertical pipe is the connection between a fixture drain and the horizontal combination pipe, and that vertical distance may not exceed 8 feet.
• 915.2.1: the slope may be no more than 1/2 inch per foot (4%) and no less than Table 704.1.
• 915.2.2: the pipe must be at least the size in Table 915.2.2, and its horizontal length is unlimited.

• 915.2.3: the system needs a dry vent connected at any point within the system, or it must connect to a horizontal drain serving 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 vertically to at least 6 inches above the flood level rim of the highest fixture before offsetting horizontally.
• 915.2.4: the vent is sized for the total dfu under 906.2.
• 915.2.5: each fixture drain connects to the combination pipe within the distance in Table 909.1. The combination pipe serves as that fixture's vent.

Example: a lab corridor has eight floor drains (2 dfu each = 16 dfu) joining a horizontal branch. Under Table 915.2.2, 3 inches carries only 12 dfu, so use 4 inches (20 dfu). Slope it between 1/8 and 1/2 inch per foot, and provide a dry vent.

Island fixture venting (IPC 916)

• 916.1: limited to sinks and lavatories. Residential kitchen sinks with a dishwasher connection, a food waste disposer or both may use it.
• 916.2: the vent connects to the fixture drain as an individual or common vent, and rises vertically above the drainage outlet of the fixture before offsetting horizontally or downward. A vent or branch vent for several island fixtures extends to at least 6 inches above the highest island fixture before connecting to the outside vent terminal.
• 916.3: the part of the vent below the fixture's flood level rim is installed like drainage piping (Chapter 7), but sized as a vent (906.2). Its lowest point connects full size to a vertical drain or to the top half of a horizontal drain. Cleanouts must allow rodding of all vent piping below the flood level rim, and two-way rodding through a cleanout is allowed.
• 904.1.1: an island fixture vent cannot serve as the one required outdoor vent for a building drain.

Single-stack vent systems (IPC 917)

The drainage stack and branch piping act as the vents.

• Table 917.2: stacks sized by height. A 3-inch stack under 75 feet carries 24 dfu, and a 4-inch stack 225 dfu. A 3-inch stack serves no more than two water closets, and the stack vent is the same size as the stack.
• 917.3: no more than one water closet may discharge to a 3-inch horizontal branch within 18 inches horizontally of the stack.
• 917.4: water closet connections are no more than 4 feet from the stack (8 feet with a sanitary tee), and other fixtures no more than 12 feet.
• 917.5: vertical fixture-drain piping is 2 inches (3 inches for water-supplied urinals and standpipes), with a maximum drop of 4 feet.
• 917.7 and 917.8: no fixture connections within 2 feet above or below a horizontal offset. Stacks over two branch intervals may not receive branches from the lower two floors.

Air admittance valves (IPC 918)

An air admittance valve (AAV) opens to let air into the drainage system when pressure drops, and closes by gravity to seal against sewer gas. It cannot relieve positive pressure.

• 918.1 standards: stack-type AAVs meet ASSE 1050; individual and branch-type AAVs meet ASSE 1051.
• 918.2: install AAVs after the DWV water or air test (312.2 or 312.3).
• 918.3: individual, branch and circuit vents may end at individual or branch-type AAVs. Stack vents and vent stacks may end at stack-type AAVs.
• 918.3.1: individual and branch AAVs vent only fixtures on the same floor connected to a horizontal branch. Where that branch is more than four branch intervals from the top of the stack, it needs a relief vent to a vent stack, stack vent or the outdoors. The relief vent connects between the stack and the most downstream fixture drain.
• 918.3.2: stack-type AAVs may not terminate vent stacks or stack vents serving drainage stacks with more than six branch intervals.
• 918.4 location:
  • individual and branch-type AAVs at least 4 inches above the horizontal branch drain or fixture drain being vented;
  • stack-type AAVs at least 6 inches above the flood level rim of the highest fixture being vented;
  • within the maximum developed length allowed for the vent;
  • at least 6 inches above insulation materials.
• 918.5: AAVs need access and must be in a location that lets air enter the valve.
• 918.6: an AAV must be rated for the size of the vent it connects to.
• 918.7: at least one stack vent or vent stack in each plumbing system must extend outdoors to the open air.
• 918.8 prohibited installations:
  • in nonneutralized special waste systems, unless the AAV meets ASSE 1049, is made of approved chemical-resistant material, and is tested to ASTM F1412;
  • in spaces used as supply or return air plenums;
  • to vent sumps or tanks, unless the sump or tank vent system was designed by an engineer;
  • on outdoor vent terminals just to reduce clearances to air intakes.

Engineered and computerized vent systems

919 allows engineered vent systems that meet the design and approval rules of Section 316, and 920 allows computerized vent design that keeps pressure changes at traps within limits.