6.3 Air Admittance Valves (AAVs) & Wet Venting

Traditional venting relies on an open pipe to the outdoors, allowing air to flow freely in both directions. An Air Admittance Valve (AAV) is a mechanical device designed to allow air to enter the plumbing drainage system when negative pressure develops (preventing trap siphonage), but closes by gravity and seals tightly when the pressure equalizes or becomes positive (preventing sewer gases from escaping into the room). AAVs have revolutionized plumbing design, especially in complex renovations or island sink installations, by eliminating the need to run costly and intrusive dry vent piping all the way to the roof.

AAV Installation Limits and Locations

While AAVs are widely permitted in the IRC, their use comes with specific restrictions and installation criteria. First, an AAV must be installed in an accessible location. Since it is a mechanical device with a moving seal, it may eventually fail and require replacement. Hiding an AAV behind drywall without an access panel is a strict code violation. Common acceptable locations include under the sink cabinet or inside a dedicated, accessible wall box. Second, the AAV must be located within a space that allows air to enter the valve. If an AAV is sealed in a completely airtight box, it cannot draw the air needed to protect the trap. Third, the AAV must be installed a minimum of 4 inches above the horizontal branch drain or fixture drain being vented. This elevation protects the delicate internal membrane of the valve from being fouled by wastewater during a temporary backup or slow drain condition. If an AAV is installed in the attic to vent a stack, it must be located a minimum of 6 inches above the attic insulation to ensure the air intakes are not blocked by fiberglass or cellulose.

Relief Vents and AAV Limitations

AAVs only relieve negative pressure; they do not allow positive pressure to escape. Because drainage systems generate positive pressure when water falls down stacks and displaces air, a system entirely vented by AAVs would fail. Therefore, the IRC mandates that in every building, at least one main vent must extend to the outdoors to relieve positive pressure. Additionally, AAVs cannot be used to vent a sewage ejector sump or a chemical waste system, as the positive pressure from the pump or the corrosive fumes from chemicals would damage the valve or escape into the building.

Horizontal Wet Venting

Wet venting is a clever and efficient design method where a single pipe serves as both the drain for upper fixtures and the vent for lower fixtures. Because the pipe is oversized, wastewater flows in the bottom portion of the pipe, while air flows in the upper portion. Under the IRC, any combination of fixtures within two bathroom groups located on the same floor level can be vented by a horizontal wet vent. A bathroom group typically consists of a water closet, lavatory, bidet, and a bathtub or shower. The wet vent connects the fixtures, and at the most upstream point, a dry vent is required to supply air to the entire wet-vented system.

Sizing the Wet Vent

The size of the wet vent is determined by the total Drainage Fixture Units (DFUs) discharging into it, using the specific wet vent sizing table (IRC Table P3108.3). The minimum size for a wet vent is:

• 1 DFU = 1-1/2 inch
• 4 DFUs = 2 inch
• 12 DFUs = 2-1/2 inch
• 32 DFUs = 3 inch

A common layout involves a bathroom where the lavatory drain serves as the wet vent for the shower and the water closet. The lavatory's 1-1/2 inch trap arm connects to a 2-inch vertical pipe. The upper portion of this vertical pipe continues up as a dry vent. The lower 2-inch portion acts as the drain for the lavatory (1 DFU) and the wet vent for the downstream fixtures. This 2-inch pipe is sufficient to wet vent up to 4 DFUs, which perfectly accommodates a shower (2 DFUs) and the lavatory itself. Once it connects to the water closet, the piping must typically increase to 3 inches for the toilet drain.

Vertical Wet Venting and Waste Stack Venting

Vertical wet venting is similar in principle but applies to vertical pipes. A vertical wet vent can serve fixtures on multiple floors, provided the pipe is correctly sized to handle the cumulative waste without blocking the airflow. Waste stack venting is another specialized method where a vertical stack that receives only liquid waste (no water closets) serves as the vent for the fixtures connected to it. The waste stack must be oversized—for instance, a stack receiving a washing machine, kitchen sink, and bathroom sink may need to be 3 inches in diameter to ensure the falling wastewater never forms a full cross-sectional slug that would trap air.

Proper Connection of Wet Vents

The critical rule for all wet venting is that the dry vent must connect to the wet vent upstream of all fixtures being wet vented. If a fixture discharges into the wet vent upstream of the dry vent connection, it can choke off the air supply for the rest of the bathroom group. The water closet, being the largest surge of waste, must always be the most downstream fixture on a horizontal wet vent. This prevents the heavy solid waste from washing past the smaller fixture connections and potentially clogging the air pathway.