4.5 Air Admittance Valves, Combination Waste & Vent, and Island Venting

Air Admittance Valves, Combination Waste & Vent, and Island Venting

Air Admittance Valves (AAVs) & Iowa Mechanical Vent Amendments

An Air Admittance Valve (AAV) is a one-way pressure-activated mechanical valve engineered to admit ambient room air into a drainage pipe when negative pressure occurs during fixture discharge. When drainage flow stops and pressure inside the pipe equalizes to atmospheric pressure or turns positive, the internal sealing assembly drops back down by gravity, sealing sewer gases inside the DWV system.

 ┌─────────────────────────────────────────────────────────────┐
 │               AIR ADMITTANCE VALVE (AAV)                   │
 │                                                             │
 │   [ Negative Pressure (< -0.05" W.C.) ] ──► Diaphragm Lifts  │
 │                                              (Air Admitted) │
 │   [ Neutral / Positive Pressure ]       ──► Diaphragm Seals  │
 │                                              (Gas Blocked)  │
 └──────────────────────────────┬──────────────────────────────┘
                                │
                                ▲ Min. 4" Above Trap Arm
                                │
                         Trap Arm Riser

Standards and installation rules

1. Manufacturing standards. Individual and branch-type air admittance valves conform to ASSE 1051. Stack-type air admittance valves conform to ASSE 1050.
2. Location. Individual and branch AAVs are installed not less than 4 inches (102 mm) above the horizontal branch drain or fixture drain being vented. Stack-type AAVs are installed not less than 6 inches (152 mm) above the flood level rim of the highest fixture being vented. In attics, an AAV is installed not less than 6 inches above the insulation.
3. Within the permitted vent length. An AAV must sit within the maximum developed length permitted for the vent it terminates, and it must be rated for the size of that vent.
4. Access and air supply. AAVs must be accessible and located where air can actually reach the valve. A sealed, foamed-in wall cavity starves the valve and defeats it.
5. At least one open vent is always required. An AAV opens on negative pressure and seals under neutral or positive pressure, so it can relieve a vacuum but can do nothing about the positive pressure that builds at the base of a stack. Within each plumbing system, not less than one stack vent or vent stack must extend outdoors to the open air.
6. Install after testing. AAVs are installed after the required DWV test has been performed; a valve left in place during a 10-foot head or 5 psi air test will be destroyed.
7. Prohibited installations. AAVs shall not be used to vent sumps or tanks unless the vent system for that sump or tank has been designed by an engineer, and shall not be installed on outdoor vent terminals for the sole purpose of reducing clearances to gravity or mechanical air intakes.
8. Not for chemical or special waste. AAVs are not used on chemical waste, acid waste, or similar special drainage systems.

[!IMPORTANT] Check the local rule before you specify one. Iowa's amendments in rule 481—425.4(7) address Sections 901.1, 906.1, and 908.2.2 and do not add a state AAV allowance or restriction, so the adopted UPC text and the AHJ govern. Because permit, inspection, and enforcement in Iowa are administered locally, a city may be stricter about where AAVs are accepted. Ask before you rough in.

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Combination Waste and Vent (CWV) Systems (UPC Section 910.0)

A Combination Waste and Vent (CWV) system is a specially engineered DWV design in which horizontal drain pipes are intentionally oversized so that liquid waste flows along the bottom invert while atmospheric air flows freely along the upper crown of the same pipe. This eliminates separate dry vent piping.

Allowed vs. Prohibited Fixtures

Mandatory Design Parameters for CWV Piping

1. Two nominal pipe sizes larger Rule: Every CWV pipe segment must be sized at least two (2) nominal pipe sizes larger than the standard minimum drain size required by DFU tables (e.g., a floor drain requiring a 2-inch drain must be piped using a 4-inch CWV pipe).
2. Shallow Slope Limits: CWV horizontal drains must be installed at a slope of not less than 1/8 inch per foot ($1%$) and not more than 1/2 inch per foot ($4%$). Steeper slopes cause liquid to fill the pipe cross-section, destroying the air space.
3. Air Relief Venting: A CWV system must connect to a dry vent line at its upper terminus to ensure continuous air circulation.

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Island Sink Venting Systems (UPC Section 909.0)

In kitchen islands and barber shops, sinks are positioned in open floor plans without wall cavities to run vertical vent pipes 6 inches above the flood level rim. UPC Section 909.0 governs the special island sink venting loop architecture.

                    ┌──────────────────────┐
                    │ Under-Counter Loop   │  <-- Highest point under counter
                    └─┬──────────────────┬─┘
                      │                  │
     Sink Outlet ─────┼── Trap Arm       │
                      │                  │
                      ▼                  ▼
                Vertical Waste     Vertical Foot Vent Riser
                Riser              (Drops below floor)
                      │                  │
    ══════════════════╪══════════════════╪════════════════════ Floor Line
                      │                  │
                      ▼ Cleanout         ▼ Cleanout
               Drain Branch ─────── Foot Vent (Sloped to drain)
                                         │
                                         ▼ To Main Vent Stack

Component Requirements for Island Loop Vents

1. Under-Counter Loop Bend: The trap arm connects to a vertical waste riser. The pipe extends vertically upward as high as possible under the countertop and loops 180 degrees using two 90-degree elbows or a return bend.
2. Foot Vent Riser: From the loop bend, a vertical vent pipe drops down below the floor as a "foot vent."
3. Horizontal Foot Vent Run: Below the floor, the foot vent runs horizontally, sloped downward toward the drain line at 1/4" per foot, and connects to the main vertical vent stack.
4. Mandatory cleanout. UPC Section 909.1 requires an accessible cleanout installed in the vertical portion of the foot vent. The section also requires drainage fittings on the vent below floor level, a slope of not less than 1/4 inch per foot back to the drain, and a return bend under the drainboard that is either a one-piece fitting or an assembly of a 45-degree, a 90-degree, and a 45-degree elbow in that order. The island sink drain upstream of the returned vent shall serve no other fixtures.