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.
I have exactly your problem, and bet Gary’s answer did not help. My AAV is at top of sink height or about 14″ above 11/4″ horizontal drain pipe. First AAV ($6) did not open, discarded. 2nd AAV ($26 branded) works but not well, particularly when Dishwasher drains. DW is installed with the high loop method, no air gap. My question to Gary is since one AAV proved better, is there any AaV that may open with less “suction”?
I’d need to see the configuration of the pipe to give more information.
Had a sink that started no-slow draining, I though some one must have poured thick grease or white glue down the sink. After $40 for drain opener, special plunger, and snake, no joy after two days. I opened the shop cabinet and eureka found a AAV, took it drained perfect, I could not believe it. Im not a plumber but a refrigeration tech, we do the plumbing as it relates to our fixtures. Learned something new today. Glad I found this blog.
Our 1-story + basement home in Michigan is now 4 years old, and we have been experiencing sewer smell inside the home throughout the seasons. Every sink in the home is equipped with an AAV and in total there are 10 AAVs. My first solution was to have septic tanks cleaned and checked, and a local tradesman completed this task and indicated the tanks were in good condition. I have now started to simply replace the AAV in the areas where the smell is identified. The source of smell seems to move within the areas as I replace the AAVs. I have now replaced 5 AAVs with one bathroom sink where the smell will simply not go away. Reading the many reviews about how AAVs are suppose to last much longer than 4 years, I assume there must be something else going on with the plumbing in our home. We would appreciate if you could share any other tips for solving this mystery, other than replacing AAVs.
Hello Heraldo
A lot of times when you have a drain which has a slow vent it will siphon or suck the liquid out of the trap of one or more of your other sinks or basins. This can in turn give you a odor coming up from the sewer. I hope this helps….
Do you have at least one vent that extends through the roof?
From the arrival above: A minimum of one vent pipe shall be extended to the open atmosphere for each building drainage system to relieve 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 an open-air vent is recommended, not required, to be located as close as possible to the building drain and building sewer connection. Check your vent pipe for loose connections/joints. Sewer gas leaks can be a challenge to find. If you know a home inspector in your area, ask them to bring a gas detector for a specific area inspection.
I have 2 AAV’s installed in an outside vented attic space above an unfinished bath whereby I need to reroute the pipes going up inside of non load bearing interior 2×4 framed walls from the crawl space as am moving or eliminating the walls.
The floor has in-floor heat lines so I cannot just drill a vent stack pipe just anywhere & the attic space is seal-ed up all but that exterior grilled attic vents with no access space provided to get to the attic due to the type of trusses & steep roof. Both loft walls, 1 with with wood tongue & groove and the other with an added large closet would have to be demoed out to get to each attic space area. Really not the greatest design.
Is it possible I add up to at least 2-90’s to each 2″ ABS piping to get it back up to that same attic space that is topped off with an AVV in each location. I can come up other framed walls to get to that same attic location.
The Question is, is there a limit to the number of 90 degree ABS fitting/ elbows that I can add to that vertical pipe during its rerouting process.
One pipe appears to be for the not-yet-installed walk in shower location & the other comes off the main drain line that which leads out to a septic system but also T’s to that drain pipe for a future garden tub area.
Thanks for any and all advise
You have a lot going on there – that’s a lot of moving parts – I’d need to “see” the configuration to give more advice.