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.
The Pros
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.
The Cons
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
Home Inspector – Home Builder and Building Consultant/Coach
Hi Gary…thanks for this informative post…I just have a few questions about AAVs in the basement when there is no venting options available..can every fixture have its own AAV?
Thank you fro this informative post. My daughter owns a home in Charlotte,NC and could benefit from your knowledge of AAV’s as she has new hot water heater and plumbing is “loud” at times of use. Her home has a master bathroom with shower,toilet a soaker tub, and 2 sinks;next to the original bathroom with a wall shower/tub, toilet ,and sink.What do you think?
Thank You for your help.
Krystal Grooters
Nice tutorial
I’m a licensed home inspector in Texas. I have ran across a situation that I can’t find a clear answer -situation I inspected a spray foamed metal frame house- all plumbing vents terminate in the attic with AAV installed. The way I interpret the IRC P3102.1-(required vent extension) says at least 1 plumbing vent stock must terminate outside- Do u know if any new plumbing code that allows all plumbing vents to terminate in the attic with AAV devices installed. Thank you for your time.
no vent shall terminate in an attic unless it is well ventilated and/or is working independently (atmospherically) of the house (this is very hard to do). the very essence of a vent is to prevent siphon and vacuum forces in building up in the system, it is also to provide an exit for the gases produced in the system. The AAV’s only function is to avoid costly installation of complicated vent system for isolated/difficult fixtures.
I would like to know the answer to Trent Spivey’s question — is there a new plumbing code that allows for all plumbing vents to terminate in the attic with aav devices installed.
Trent , Alison – When using AAV – we need at least one vent from the plumbing system to push through the roof.
“Within each plumbing system, not less than one stack vent or vent stack shall extend outdoors to the open air.”
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I have a kitchen sink with a AAV tied into a 1.25” horizontal drain line after the trap. The top of the vent is about 1” above the bottom of the sink. The drain has loud gurgles two or three times a day whether the sink has been used or not. Does the height of the vent make a difference? Is the gurgling coming from poor drainage from the trap?
Thanks in advance for your help.
Dave
Your sink may be noisy due to a tied in drain line, maybe from reverse osmosis or the like. We had that problem.
Your gurgling is probably coming from positive pressure pushing air through your trap. Aav’s Are only to let air in, not out. Flush a toilet and listen at your sink, if it gurgles that is what is happening.
Size your drain pipe up to 1½ or 2 inch.
We recently added another story to our home and one of the vents wasn’t extended to the roof. They offered to cap it and put on an AAV. This seemed like a bad solution shortcut to me, but the alternative is a lot of drywall dust and disruption in my house. Is it reasonable, assuming an AAV is up to local codes, to do this, or is it just a hack to get around a mistake? Thanks!
If you add an AAV (most brands work great) leave access to maintain the vent. It’s mechanical – and all mechanical parts eventually wear out…and Yes, check with your local code office – some jurisdictions won’t allow an AAV.
I have an kitchen island with an AAV. When you drain a large amount of water, like a sink 1/2 full, it backs up into the other basin and the sink drains slowly. If I remove the AAV while the sink is draining, it drains 10X faster. I can feel the vacuum when I remove the valve. I have tried replacing it with another brand but still the same slow drain. Remove the AAV and the water rushes away. Any ideas?
Brendt – quick question. Is the top of your current AAV below the bottom of the sink level? If so – try routing the stack under the sink to an area where you can get the top of the AAV higher (as close to the underside of the countertop as you can). Obviously you’ll want to leave enough room to remove and maintain the valve (in case you need to replace it). The important aspect of using an AAV: Most local plumbing codes require at least one vent in the plumbing system to terminate through the roof. As with any work you do – if in doubt, contact the local code officials for specific guidance.