Most residential building drainage systems are vented directly through the building’s roof. Here’s an example. See the white pipe on the roof at left. The plastic pipe is “flashed” or sealed at the roof penetration to prevent rainwater from leaking into the building.
During maintenance you’ll want to be assured the top of the vent is kept open. A blocked vent can be caused by leaves, dead animals and ice dams (in very cold climates). Another “not-so-common” way the vent can become clogged is when a horizontal section of the venting system itself (inside the attic) is sloped the wrong way. When that happens it can fill with rain water or condensation. Experienced home inspectors should be on the look out for these conditions.
Air Admittance Valves (AAV)
Sometimes we can’t (or don’t want to) vent through the roof line.
In these rare cases plumbers use an Air Admittance Valve. Air Admittance Valves (aka Studor vents) are “negative-pressure-activated” one-way mechanical valves. Used most commonly at an island sink or vanity the vents are also located in the attic to prevent the roof penetrations (as seen above) on the front roof line elevation. Think about it – when have you ever seen a plumbing vent on the front roof line?
How It Operates
If we didn’t have either venting method you would have very noisy sink and toilet drains. A discharge of wastewater down your sink drain causes a valve on the AAV to open. When it opens air is allowed to enter the plumbing system. Watch this video to help understand how a plumbing vent works.
An AAV can significantly reduce the amount of venting materials needed in a plumbing system. That’s money in the pocket for the plumber. They also allow greater flexibility in the layout of plumbing fixtures, and reduce long-term roof maintenance. If we don’t have a hole in the roof, it’s easier to maintain the roof. AAVs have been effectively used in Europe for more than two decades. However, there are a few limitations.
Some state and local building departments prohibit AAVs. Check with your local AHJ – Authority Having Jurisdiction for more info.
AAV’s are certified to reliably. However anything mechanical can and will fail. Some manufacturers claim they’re good for 500,000 uses (approximately 30 years of use).
US manufacturers offer warranties that range from 1 year to “lifetime”. You’ll want to seek out the warranty info. Most plumbers won’t have it.
Note: An AAV should should not be tampered with or spray painted. I have had plumbers tell us that sewer flies and bugs have been seen at failed air admittance valves. So if you detect a sewer odor under and around your sink or in the attic, the AAV could have let you down.
Inspecting The Studor AAV
- STUDOR AAVs must be located a minimum of four (4”) inches above the horizontal branch drain or fixture drain being vented.
- STUDOR AAVs shall be accessible should replacement be required. For in-wall installation, use STUDOR recess box/grill combination.
- STUDOR AAVs location must allow for adequate air to enter the valve. When located in a wall space or attic space lacking ventilation openings, opening shall be provided. Locating the valve in a sink or vanity cabinet is acceptable.
- STUDOR AAVs must be installed in the vertical, upright position. A maximum deviation (in either direction) for plum of 15 degrees is allowed.
- The vent shall connect to the drain vertically to maintain an unobstructed opening in the piping to the STUDOR AAVs.
- A minimum of one vent pipe shall extended to the open atmosphere for each building drainage system for relief of positive pressure, the size of this vent is not specified because this single vent does not determine the total amount of aggregate cross-sectional area of the vent system. The total amount of the cross-sectional area of vents combined on the system has to equal the aggregate cross-sectional area of the building drain. When properly installed an air admittance valve in the system is equivalent to an open vent pipe having the same cross sectional area as any other vent. Such open air vent is recommended, not required, to be located as close as possible to the connection between the building drain and building sewer.
- The Maxi-Vent® must be installed six (6”) inches above the highest flood level rim of the fixtures being vented in stack applications.
- STUDOR AAVs installed in attic area must be located a minimum of six (6”) inches above the ceiling insulation.
- The use of Tec-Vent® in return air plenums shall be allowed only in engineered drainage systems designed by a design professional and approved by the local authority.
- The maximum height of drainage stack being vented by a MAXI-VENT must not exceed six (6) branch intervals unless it is used in conjunction with a stack that is connected to a P.A.P.A. and AAVs on the branches.
- When a horizontal branch connects to a stack more than four (4) branch intervals from the top of the stack. A relief vent shall be provided. The relief vent must be located between the connection of the branch to the stack and the first fixture connecting to the branch. The relief vent may also serve as a vent for the fixture. The relief vent must connect to the vent stack, stack vent or extend outdoors to the open air unless it is used in conjunction with a stack that is connected to a P.A.P.A. device and AAVs on the branches.
- Only Teflon® tape can be used on the valves’ threads. Use of primer, solvent cement or pipe dope will void the STUDOR warranty.
- The Redi-Vent®, Mini-Vent®, Maxi-Vent®, Tec-Vent® and Chem-Vent® must be installed at finish, after the system rough-in and pressure test.
- Air Admittance Valves may be used on grease traps as long as they are not subject to positive pressure.
- Other Information – Studor PDF
NAHB Research Center video showing the basic principals of the AAV