Stumped by vent issue with AAV in new bathroom

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ANewHouse

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Hi, first post, but I've been casually reading for a while. I've found a lot of good advice here - thanks already!

I'm working on a bathroom, and I'm totally stumped by a vent issue.
Not sure about the best way to attach pictures, so I've uploaded and pasted links below.
There just isn't a good way to get through the roof from this room, so I have an Air Admittance Valve under the sink. After the P-trap, the drain goes into 2" PVC. About 5 feet downstream of that, there's a toilet, which drains into 3" PVC. Then the 3" PVC runs across a room to the main stack with roof vent, which is about 16' away. All is sloped properly so water doesn't pool in the drain lines. The AAV under the sink (rated for 20 DFU) should be adequate to vent both fixtures. However, when the AAV is attached, they don't vent! The toilet doesn't flush all the way (bowl fills up and then drains extremely slowly). When I run the sink wide open for more than a few seconds, it drains slowly and eventually causes the toilet to bubble. However, if I reach under the sink and loosen/remove the AAV, I can feel air rushing in, and everything drains just fine (no sewer gas smell). This makes me think it's a problem with the AAV itself, and not a problem with the rest of the plumbing. So I ordered a different brand of AAV, and the same thing happens! (I've now tried both Oatey and Studor AAVs).
Any ideas? Thanks in advance!

plumbing schematic.jpg plumbing sink photo.jpg

https://drive.google.com/open?id=0B4omjtbz0P0acnNEcTJKQnRRVUd3Rnk3Zk8wVjFuZlpHRUln

https://drive.google.com/open?id=0B4omjtbz0P0acnBoYTZFRmtKZmQzODYzSFdfRHgxTGRaRUtj
 
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Reach4

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Put some light plastic sheet over the open AAV fitting. Does the plastic balloon out when you do a flush? If so, you need a real roof vent somewhere. Does the main stack extend up through the roof? I am not a plumber.
 

Gary Swart

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AAV require a roof vent in the system. Your post doesn't say if there is one elsewhere in the house or not.

oatey-sure-vent.jpg
 
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ANewHouse

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AAV require a roof vent in the system. Your post doesn't say if there is one elsewhere in the house or not.
Sorry I didn't specify - yes, there is a roof vent above the "main stack" I mentioned. I'll edit the OP accordingly.
 

ANewHouse

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Put some light plastic sheet over the open AAV fitting. Does the plastic balloon out when you do a flush? If so, you need a real roof vent somewhere. Does the main stack extend up through the roof? I am not a plumber.
Good thought to confirm which direction the air is moving - I'll try that this evening. It seems like it's pulling air in when I remove the AAV, but I haven't tested like this to be sure.
 

hj

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It is probably blowing out because of some sort of obstruction downstream. An AAV ONLY allows air in, and if there is a positive pressure, which it can do nothing to eliminate, the drains will not function.
 

ANewHouse

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It is probably blowing out because of some sort of obstruction downstream. An AAV ONLY allows air in, and if there is a positive pressure, which it can do nothing to eliminate, the drains will not function.
Nope - just tested it as per Reach4's suggestion, with the AAV removed and a plastic bag over the end of the pipe. It's definitely pulling air into the pipe when I flush the toilet or run the sink.
Also, there are no major obstructions downstream, because everything drains just fine when I remove the AAV altogether.

So why isn't either of my AAV's doing its job? Both of them still seem to function when I try to blow into them (seals tight) or suck air through them (opens easily). Why don't they do the same thing when threaded onto pipe?
 

Reach4

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I have a hard time seeing how the toilet would have a hard time flushing due to a vacuum in the pipe. I was sure suspecting a pressure that was fighting the water flow during the flush.

The AAV under the sink (rated for 20 DFU) should be adequate to vent both fixtures.
http://www.oatey.com/products/air-admittance-valves/aavs/sure-vent-air-admittance-valve-160-dfu-capacity says
160 DFU Capacity
2" – 3"​

So if the probem is a vacuum in the pipes, maybe a different unit would give you extra air.
http://www.oatey.com/products/air-admittance-valves/aavs
 
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ANewHouse

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http://www.oatey.com/products/air-admittance-valves/aavs/sure-vent-air-admittance-valve-160-dfu-capacity says
160 DFU Capacity
2" – 3"​

So if the probem is a vacuum in the pipes, maybe a different unit would give you extra air.
http://www.oatey.com/products/air-admittance-valves/aavs
Maybe... It'd be a little tricky to fit the bigger AAV under the sink, but if it will help, I'm willing to try it! However, it seems to me (based on my limited understanding of AAVs) that a larger unit would just allow more air, but I don't know that it would open any easier. If the smaller units were opening, but just not venting fast enough, I think a bigger unit would solve the problem. In my case, the smaller AAVs aren't opening at all, so would a bigger one help?
 

Reach4

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How about this: instead of a flat piece of 1 mil plastic, put a bag, such as a plastic bread bag over the hole and retain with tape or rubber band. Then flush. I am wondering if the bag will inflate slightly. I am hoping that would be more sensitive to detect a pressure than a taut piece of plastic.
 

ANewHouse

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How about this: instead of a flat piece of 1 mil plastic, put a bag, such as a plastic bread bag over the hole and retain with tape or rubber band. Then flush. I am wondering if the bag will inflate slightly. I am hoping that would be more sensitive to detect a pressure than a taut piece of plastic.
That's exactly what I did! Held a plastic grocery bag over the pipe where the AAV goes. Flushed, and the bag was noticeably tugged into the pipe. No ballooning outward.
 

hj

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something, such as a double trap, WILL impede flow even with an AAV, but allow perfect drainage with the AAV removed. THAT is your situation, regardless of what your paper test shows, and NO AAV, regardless of its size, or even quantity, will change that.
 

Reach4

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ANewHouse, that is really baffling. As far as a vent for the water leaving the toilet, the room itself is a really big vent.

I suspect a better-flushing modern toilet would not display this symptom, but because I can't understand why you get this symptom, I can't be sure.
 

ANewHouse

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something, such as a double trap, WILL impede flow even with an AAV, but allow perfect drainage with the AAV removed. THAT is your situation, regardless of what your paper test shows, and NO AAV, regardless of its size, or even quantity, will change that.
When you say "double trap," do you just mean two fixtures with their own traps that are using the same vent? And you're suggesting the only solution is a roof vent? Just want to be sure I understand.
 

ANewHouse

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ANewHouse, that is really baffling. As far as a vent for the water leaving the toilet, the room itself is a really big vent.

I suspect a better-flushing modern toilet would not display this symptom, but because I can't understand why you get this symptom, I can't be sure.

Thanks Reach4 - glad I'm not the only one who's baffled! The toilet is brand new. (Well, it's about 1 year old, but never been installed until now. It's this model from Lowes. I have the same kind, but standard single flush, in my other bathroom, and it's worked great for 3 years.)
I guess it's possible there's a problem with the toilet; I hadn't considered that. But I keep coming back to this - why would it work fine, and pull air as it's supposed to, with the AAV removed?
 

ANewHouse

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something, such as a double trap, WILL impede flow even with an AAV, but allow perfect drainage with the AAV removed. THAT is your situation, regardless of what your paper test shows, and NO AAV, regardless of its size, or even quantity, will change that.
Sorry for the double reply... If a double trap (meaning two fixtures with traps) is the issue, would it help to install two AAVs; one for each fixture?
 

hj

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A "double trap" is two traps on the SAME drain line which traps air between them and the only way to let it out is to unscrew the AAV. The "second trap" is not always a conventional trap, but rather a sag in the line that holds enough water to block the pipe, or even a flooded septic tank that has backed up water into the pipe, which will do the same thing.
 

ANewHouse

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A "double trap" is two traps on the SAME drain line which traps air between them and the only way to let it out is to unscrew the AAV. The "second trap" is not always a conventional trap, but rather a sag in the line that holds enough water to block the pipe, or even a flooded septic tank that has backed up water into the pipe, which will do the same thing.
Thank you for the explanation! I appreciate your time.
Still trying to figure this out - where would the "traps" (or flooded sections) have to be relative to the AAV in order to cause this symptom? In other words, would the AAV have to be located between the traps, or upstream of both? Is there any solution (e.g. additional AAV elsewhere in line) besides an open-air vent to the outside? I really don't see how any of these lines could have sagged enough to flood - I watched the installation as it went in last year, verifying solid pipe support and good slopes with my own eyes. I just finished drywall & flooring a few months ago, at which point everything still looked fine.
Shouldn't be a septic backup issue - this is on the second floor of a house, and everything on the main floor drains just fine.

The most reasonable place to tap into the existing drain line is indicated by the red arrow in the attached pencil drawing. If it might help, I can add an additional (higher DFU?) AAV at this location. (Not quite to scale - this would be almost immediately behind the toilet, and I would set the AAV up higher than the sink). It would be accessible from a stairway adjacent to this bathroom. If absolutely necessary, I could also add a vent to the outdoors at this location, but it would exit the house horizontally through a west-facing dormer gable wall... Exposed to wind, and more visible than I'd like. I suppose I could wrap it up vertically to extend it over the existing roof line (as in the attached sketch), but that would be even more visible and look even worse IMO. Note this is not to scale - vent would be >10' from windows, and about 20' above ground.
But I'd much rather avoid this work and any unnecessary holes through the house! Still hoping to figure out why the AAV is not functioning like it's supposed to.
Thanks in advance for any other ideas!

plumbing schematic new AAV.jpg Gable wall vent.JPG
 

Jadnashua

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Is there for chance a trap underneath the toilet? The toilet itself has an internal trap, and won't work properly if there's one underneath it.
 

hj

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quote; Shouldn't be a septic backup issue - this is on the second floor of a house, and everything on the main floor drains just fine.

IF there is NO "venting" between your AAV and wherever the "obstruction" is, it makes no difference WHICH floor it is on. REmoving the AAV would NOT "cure" a trap under the toilet. Installing an "atmospheric vent" would NOT cure the problem, it would just eliminate the symptoms. you can put in as many AAVs as you want, but until you install one that lets air OUT, they will not help.
 
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