# Air admittance valves on roof stack vents for odor control?



## semipro (Nov 23, 2017)

I'm building a deck on top of our house and would like to find a way to deal with odors from our plumbing roof stack vents. I'm wondering if I could install air admittance check valves on the roof vents to prevent odors from exiting at our roof.  These valves would still allow air to enter the stacks for proper operation of drains. 

Of course I'd need to provide a vent elsewhere for gases produced in the septic tank.  I thought I'd do this at the tank itself and could maybe use black poly to run a vent up a nearby tree to get the odors well above the ground. 

This plan also appeals to me because sometimes sewer gases exiting the roof enter windows below in the summer. 

Can anyone see any problems with this plan?


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## heat seeker (Nov 23, 2017)

I can't offer any advice, but am watching this thread. I have a similar problem.


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## Buzz Saw (Nov 24, 2017)

Couple things that come to mind
-Winter operation, would it freeze shut
- Is an AAV offered in 3"or4"
- Is it possible to relocate the vent to the other side of the house/ down wind based on prevailing winds?

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## semipro (Nov 24, 2017)

Buzz Saw said:


> Couple things that come to mind
> -Winter operation, would it freeze shut
> - Is an AAV offered in 3"or4"
> - Is it possible to relocate the vent to the other side of the house/ down wind based on prevailing winds?
> ...


Good thought on the freeze up. I could put a cap on it to keep out precipitation if problematic. 
AAVs are offered in larger sizes and rated by the number of drained fixtures they serve. (DFUs)
I can't really relocate.  There are 3 stacks in different places. If there was just one stack I'd consider just putting an extension on it when we were up on the roof deck.


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## Hogwildz (Nov 24, 2017)

Without letting the air vent outward, you may create causing gurgling & such at the drains, venting air inward instead of outward. You want the nasty gases to vent outward. Your best bet is relocate the vents prior to deck installation.


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## Sodbuster (Nov 24, 2017)

I would re-route the vents, taking into account your prevailing winds. You may be able to minimize the odor, but not completely eliminate it.


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## heat seeker (Nov 25, 2017)

I had a similar problem years ago and far away. The vent system had two stacks, and the wind would cause the fumes to come out of the downwind stack. I cured it by putting aluminum foil over one of them, with a small hole in it. End of problem. I don't know why the two stacks were connected, maybe a plumbing fluke in the house. Seems like one stack would have worked if they were interconnected. I dunno...


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## semipro (Nov 25, 2017)

Rerouting doesn't seem like an option given the circumstances. We have cathedral ceilings so no attic space and the 3 vents surround the deck area.  Above view below.  The vents are shown by the red marks and the deck is shown in white.




Below is an average wind rose diagram for our location.  Though we do have some prevailing winds, my many hours of time on the roof demonstrates that higher winds and the resulting turbulence tends to disburse the odor.  The smell is most noticeable during very low speed wind conditions.  Indeed we smell it through open windows only on the stillest nights.




WRT adding the AAVs; it seems to me that anything coming out of the vents is either off-gassing from the septic tank or due to flushing/draining.  During a draining event air enters a stack to prevent negative pressure from inhibiting water flow down the pipes.  Some air must also exit the stacks because air introduced by flushing has to go somewhere. Since we have multiple stacks the introduced air may exit at a stack not serving the drained fixture. 

I believe that allowing only incoming air at the stacks through installation of AAVs, and then providing a bidirectional vent at the septic tank will allow the system to operate normally. (diagram below)  I can't envision any situation where positive/negative pressure would build up and result in  house interior release of septic gas or poor draining unless there's an obstruction in the drain system or tank, which is its own problem.



This may be one of those things that I'll just need to try to verify.


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## Hogwildz (Nov 27, 2017)

Why not get the deck done and see if there is going to be a problem at all.


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## semipro (Nov 27, 2017)

I've been up on the roof a lot where the deck will be and its definitely a problem.   
We also get the smell coming in our windows sometimes when the wind is just right. 
Something about that smell really hangs with you too. Maybe its the hydrogen sulfide.


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## Buzz Saw (Nov 27, 2017)

I say try.  What's the worst thing to happen? 

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## festerw (Nov 27, 2017)

Not the cheapest solution, but seems to work.

https://www.amazon.com/dp/B00COU0K2C/?tag=hearthamazon-20


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## semipro (Nov 29, 2017)

festerw said:


> Not the cheapest solution, but seems to work.
> 
> https://www.amazon.com/dp/B00COU0K2C/?tag=hearthamazon-20


Thanks. I came across these as well.  
I wouldn't want to spend the money or climb on the roof every time these needed replacing, which could be often.


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## festerw (Nov 29, 2017)

semipro said:


> Thanks. I came across these as well.
> I wouldn't want to spend the money or climb on the roof every time these needed replacing, which could be often.



There's another brand called Odorhog that says average replacement for the carbon is about 3 years and is available for about $25.


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## zrock (Dec 3, 2017)

If you are getting smell back from your septic tanks through the vent stack of the house then their is a issue with your septic tank. If it is working properly their should be a crust on top of the fluid in the tank to stop gasses from coming back. When is the last time u had the system pumped and checked the inlet? The baffle in the inlet may have broken off allowing the flow to be to fast entering tank 

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## semipro (Dec 4, 2017)

zrock said:


> If you are getting smell back from your septic tanks through the vent stack of the house then their is a issue with your septic tank. If it is working properly their should be a crust on top of the fluid in the tank to stop gasses from coming back. When is the last time u had the system pumped and checked the inlet? The baffle in the inlet may have broken off allowing the flow to be to fast entering tank
> 
> Sent from my SM-G903W using Tapatalk


The septic tank was pumped last year and is in good condition. 
Yes a floating scum layer forms but gases that are byproducts of anaerobic digestion such as methane and hydrogen sulfide bubble through that layer and pass through the inlet and outlet tees to eventually exit the tank via the inlet pipe.  In a conventional installation the house vent stacks are the only place where these gases can escape. The smell there is normal.

Edit: Apparently some gases may exit the tank through the drain field also.


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## zrock (Dec 4, 2017)

I used to work on septic systems for a living and deal with them quite a bit in my job now and just installed a large system for another 8 homes. while it may be normal in some installs its not right. I have never once smelt gasses coming out of the vent stacks when iv been on top of these homes working. The only time iv smelt anything coming out of a vent stack is in the rv section of our park. Its always from someone not hooking up to the system properly with a sealed connection creating a vacuum in the system and pulling the smell back from the tanks once i get them to hook up properly its gone. You have a few possibilities your roof angles are creating a up draft and sucking the smell back through your system. Or depending on the age of your house their is a buildup of sludge and crap in the pipes and thats what your smelling and its not the tank. You could install a check valve on your mainline going to tank if you think its tank and that will stop any and all small coming back through. As for the buildup if thats the case iv had cleaned them before by running a hose back through the stack


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## venator260 (Dec 5, 2017)

My understanding of how septic tanks work is that they're to be oxygen free, so you wouldn't want to provide ventilation to the tank. On my new this summer septic system, there's a trap between the house and the tank, so the vent pipe I have through the roof would not work to vent the tank. There's no other place for air to get in, unless it can be pulled through the drainfield and the pump tank. 

I also discovered with said new tank that air exiting the system is beneficial. My toilet wouldn't flush until I took the lid off of the clean out before the trap. The house had several air admittance valves inside, but no vents to the outside at that time ( I added that afterwards). So not allowing air out of the way of the water may also cause problems.


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