bizarre issue involving air admittance valve

davidrt28 (zone 7)January 10, 2014

ok, I'll try to make it as much like cliff notes as possible since I tend to be wordy
1) have been cooking a lot of meatballs recently in a pressure cooker, and foolishly letting the grease residue be "washed" off in the dishwasher, assuming it would stay emulsified all the way to street. Nope.
2) was rushing with the dishes early this week and let more rice grains then usual get in there...uh oh...
3) had a backup, but it was never a non-moving one...just a major slow down. Oddly, plunging, even with the other sink drain unstoppered, always helped it move faster. I think this is part of the mystery...more in a moment

(btw kitchen does not have a disposal, home inspector told me that older houses w/iron pipes often don't)

4) tried various things...first a bit of lye because I've used that successfully before when it slowed down. Not this time. Bought one of the el-cheapo $20 snakes and got it 6' in before I couldn't go any further. That was a struggle. Pipe was crazy gunked up with black goo.

5) Used 1/2 a bottle (4X the normal dose) of Drano Buildup remover, overnight. This seemed to help, but it was still slow. Plunging made it move faster this time.

6) Notice the AAV for the first time. Wonder what the heck it is and read up on them. Determine it's an old non-code compliant spring loaded one...decide to shell out for a Studor mini-vent top of the line one the during the next HW store outing. Also buy some liquid-plumr "urgent" formula, to try caustics one more time.

7) let liquid-plumr work for 90 minutes. Put on new AAV valve.

8) STILL slow. I finally get curious about the AAV valve, not even trusting the new one, and completely remove it. Presto...suddenly the water drains like a champ! I easily get 5 gallons from various buckets down in one huge gulp. Mind you I had never tested without during the initial backup issue, because I wasn't even aware it was there the first day.
OTOH, it surely appears the buildup remover and caustics probably helped.

FFeeewww. SO, question is, WHY would the drains appear to work fine w/o the AAV, even a top of the line one, and run slow with it? The odd thing is just running the tap at medium won't clog it. You have to be emptying the dishwasher or pouring a gallon of water down as though draining pasta. Then, it will slow down about 1/2 way through the gallon...but a plunge or two will get it running again. This is w/the AAV valve. W/o the water now runs through the drain faster than it ever did before, probably because of all the buildup removal and snaking I did.

found this thread, which seems to describe my issue,333335

like the poster there, though, what I can't figure out is why there is no sign of a back up at all, when air is allowed to escape. (i.le., the AAV removed) FWIW I'm letting a few more ounces of the build up remove soak down there tonight, in case this is all still being caused by a bit of minor blockage. Maybe it will help more now that things are mostly moving again. ( after putting about 10 gallons plain water through to make sure the caustics are gone) BTW on the other side of the house where most of the plumbing is (bathrooms) there is a typical roof vent stack. I know those getting clogged can cause problems, but the timing is too close to my rice in dishwasher idiocy for me to think anything else was the cause. And all the drains in the bathrooms are working great)

This post was edited by davidrt28 on Fri, Jan 10, 14 at 1:08

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davidrt28 (zone 7)

update to bounce this down...since nobody seemed to have any ideas
after another round of build up remover, flushing with many gallons of hot water (without AAV valve), rest of the buildup remover over night, then snaking it an hour ago, the AAV valve is finally willing to let it drain without gurgling and slowing down. In other words, no longer any positive pressure. (I didn't mention that after finishing that post, I did the "bag test" on the AAV pipe, and saw that there was positive pressure, indicating blockage)

What did the "snake" catch after 12 feet? A wad of black cottony or linty material only as big as a wet cotton ball. That's all. I have no idea how that got down there as I don't use cotton balls and certainly don't put them in my dishwasher. The clothes washer drain is miles away figuratively. (other end of the house, 50' maybe, and closer to the main drain to the street, I can't believe it could have "floated up" against current?) There was much less black greasy goo this time, so it appears the build up remover was doing something.

BTW my first auger was the ubiquitous black plastic jacketed drill powered ones. I remember now I bought one about 10 years ago and it also died after 5 minutes of use. The clutch mechanism is a joke. Yesterday I bought a cobra handcranked one, it did much better.

    Bookmark   January 11, 2014 at 5:12AM
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AAV let's air in, not out.

    Bookmark   January 11, 2014 at 9:43AM
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I suspect that the main plumbing vent stack had frost buildup or similar that was slowing the outflow of air from the drain system -- which is necessary to provide room for the newly-introduced waste to enter the drain. The AAV helps prevent siphoning of the trap at your kitchen sink by admitting air if there is a low pressure (vacuum) in the drain below the trap- nothing more.
HOT water will help grease to move along. Grease in septic tanks can become almost like concrete.

This post was edited by bus_driver on Sat, Jan 11, 14 at 14:43

    Bookmark   January 11, 2014 at 12:20PM
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davidrt28 (zone 7)

"AAV let's [sic] air in, not out."

Maybe there were too many words or something. This is what I said:
"what I can't figure out is why there is no sign of a back up at all, when air is allowed to escape. (i.e., the AAV removed)"

"I suspect that the main plumbing vent stack had frost buildup"
We had the coldest weather in years on Tuesday, and I did think of that. But wouldn't that have affected the other drains? In reading various sources online, a blocked vent should cause: siphoning of sink traps and release of gas (didn't happen), toilets to gurgle oddly when flushed (nope), etc.

This post was edited by davidrt28 on Sat, Jan 11, 14 at 14:57

    Bookmark   January 11, 2014 at 2:34PM
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Venting issues can do weird things, depending on how and where air moves in the system. Sometimes a blocked vent in one location can cause a backup or gurgling somewhere else.

    Bookmark   January 12, 2014 at 12:02AM
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I have a similar issue. Aav seems to create an air blockage in my kitchen sink. What is the "bag test"? I can't find a good description/explanation. We never had an issue with this sink, but it was never properly vented. It was not tied into the main vent stack to the roof and it didn't have an aav. There was just a tall open pipe with a cloth shoved in it! We thought we were doing the right thing by installing an aav but it has caused this huge headache.

    Bookmark   May 5, 2014 at 7:17AM
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davidrt28 (zone 7)

Jennifer, tie an air-tight plastic bag around the pipe where the aav attaches. (w/o the aav present of course!) If the bag inflates when water goes down the drain, you have a drainage or venting problem.

    Bookmark   May 12, 2014 at 9:20PM
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