6.4 Air Admittance Valves: Standards, Where Permitted, Location & Prohibitions

Why this section matters

Air admittance valves (AAVs) are mechanical one-way vents. They let air into the drainage system when it is under negative pressure and seal it when pressure is zero or positive. Exam questions focus on where they are allowed, how high they must be and where they are banned, all in 2021 IPC Section 918.

How an AAV works

• At rest: a light seal (diaphragm or disc) sits closed by gravity, so sewer gas stays in.
• Fixture draining: the vacuum behind the water lifts the seal, and room air enters to protect the traps.
• Positive pressure from a downstream surge pushes the seal tighter.

Because an AAV cannot release positive pressure, the system still needs at least one open vent to the outdoors.

Standards and installation sequence (918.1–918.2)

Install AAVs after the DWV water or air test (312.2 or 312.3). Test pressure can damage the valve, and the valve can make a test look tighter than the piping really is.

Where AAVs may be used (918.3)

• Individual, branch and circuit vents may end at an individual or branch-type AAV. Stack vents and vent stacks may end at a stack-type AAV.
• 918.3.1: Individual and branch-type valves may vent only fixtures on the same floor level that connect to a horizontal branch drain. If the branch is more than four branch intervals from the top of the stack, it also needs a relief vent to a vent stack, a stack vent or the outdoors. That relief vent connects between the stack and the most downstream fixture drain, is sized per 906.2, and may vent other fixtures.
• 918.3.2: A stack-type AAV may not be the terminal for a stack vent or vent stack serving a drainage stack with more than six branch intervals.

Location, access and size (918.4–918.6)

Manufacturers also require the valve to be installed upright (commonly within 15 degrees of vertical) and kept within its rated temperature range. Follow those instructions under IPC 303.2.

The open vent requirement (918.7 and 904.1)

Every plumbing system needs at least one stack vent or vent stack that extends outdoors to the open air (918.7). Section 904.1 also requires each building drain's system to have at least one dry vent extending outdoors, sized per 906.2 from the required building drain size. The open vent relieves positive pressure from the public sewer or septic tank, which an AAV cannot do.

Prohibited installations (918.8)

1. In nonneutralized special waste systems (Chapter 8), unless the valve meets ASSE 1049, is made of approved materials and is chemical-resistance tested to ASTM F1412.
2. In spaces used as supply or return air plenums.
3. To vent sumps or tanks, unless the sump or tank vent system was designed by an engineer. Sewage ejector pits normally need an open vent sized by Table 906.5.1.
4. On outdoor vent terminals used only to reduce the required clearance to gravity or mechanical air intakes.

Typical uses

• A kitchen island sink, as an alternative to a 916 island vent, where the AHJ allows it. The AAV sits as high as possible in the cabinet, at least 4 inches above the fixture drain, within the Table 909.1 distance.
• Remodels where a new lavatory cannot reach an existing vent. The AAV must stay accessible in the vanity.
• Stack-type valves on short stacks (six branch intervals or fewer) where the building still has its required open vent.

Exam traps

• 4 inches above the drain for individual and branch valves. 6 inches above the flood rim for stack valves. 6 inches above insulation for all.
• Install after the DWV test.
• Never the only vent: at least one stack vent or vent stack must reach outdoors.
• Sumps: an AAV is allowed only on an engineered sump vent system. Plenums are never allowed.