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Old 10-19-2009, 10:58 PM   #1
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Sink gasping water


I have been in the process of remodeling our downstairs bathroom. I have recently installed the new sink and vanity (to the original's location) and the toilet (back to it's original location). The only thing that I have changed in the vent system of the pipes is the shower; it was a tub that had a normal tub vent system but I have changed it to a shower enclosure only and eliminated the vent, as the drain only goes about 3 feet into the main vent line. I asked several plumbers that I see on job sites regularly and they all said that doing it that way would probably be fine.
The shower has a p trap under it and the sink has a p trap with a Studor vent at the top of the elbow where the trap enters the DWV. The reason for the AAV is that the sink used to have a piece of flexible black pipe that was the vent. Not wanting to change the format (that sink always drained very slow before, even if I pulled my wife's hair out of it), I installed the Studor to be more proper. Its probably not code, but its not going to be inspected anyway. The toilet drains as it always has, even though it is new. I have a crawl space under this bathroom with limited access to the drains, especially the sink. I have already cut out some of the wainscoting behind the vanity to fix this. I have tried 4 different s and p trap configurations and placed the Studor AAV in two different locations. The sink continues to drain very slowly until the air bubble in the pipes is overwhelmed and then the sink gasps the water down very quickly. The shower has no drainage problems and there is never any standing water in the pan. Is it possible that the shower drain enters the main vent higher than the sink and is keeping the sink from draining? Any help is greatly appreciated.

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Old 10-20-2009, 08:57 AM   #2
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Sink gasping water


Not exactly sure what the sink problem is, but it is not "alright" to have any fixture unvented. When any drain is used, it could pull a vacuum on the shower drain and suck the water out of the trap. This will allow sewer gas into the house.

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Old 10-21-2009, 03:53 AM   #3
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Sink gasping water


The reason there is no vent other than the main line vent for the shower is this: the drain for the shower runs under the shower pan and towards the wall. At the place where the vent would be is the main line. So it empties right there and vents at the same time.

Last edited by Filbee; 10-22-2009 at 05:10 AM.
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Old 10-21-2009, 09:27 PM   #4
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Sink gasping water


just as a matter of experimentation, remove the AAV and see if the sink drains any better. If it does then we know its the AAV. If it doesnt then we have a drainaige issue to deal with.

Let us know what happens and we'll see if we can figure it out from there.
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Old 10-22-2009, 05:22 AM   #5
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Sink gasping water


I have already had the drain in 4 configurations. S trap (due to the level of the DWV pipe in the wall the p section of the trap was higher than the entry into DWV), s trap with Studor vent (in the high leg of the S, 4 inches above and away), P trap with the entry to the DWV raised 9 inches, and then finally P trap with the entry to DWV raised and a studor vent in a t joint above the DWV. The sink originally had a piece of 1 inch flexible pipe jammed and caulked into a hole cut into a street elbow at the DWV entry. The wall is very hard to work in as it is adjacent to a stairway and has cross members everywhere (this house is 104 years old). I would hate to do more to the drain system, since about 2 feet from the DWV it changes to 2 inch steel pipe. The sink is draining now better than it has with any drain configuration, but it still gasps at the water in the sink (the water will collect to about 2 inches deep before draining, at which point the drain can't get enough quick enough). The sink was pretty slow before the remodel, but there is no reason that I should accept that as satisfactory now. Also, the studor vent sits in the wall about 3 inches above the drain opening of the sink. I cannot go any higher. As far as the shower pan goes, there has never been a problem with its ability to drain. I have inspected in the crawl space directly underneath it and everything looks good. I cannot tell, however, whether the shower waste enters the main drain line above or below the sink waste. Would that make the difference?
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Old 10-22-2009, 10:44 PM   #6
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Sink gasping water


Whether the sink dumps over the shower or vice versa shouldnt make any difference. The reason I said to remove the AAV was to see if it was the cause of the problem. I've seen AAVs straight out of the box that didnt function correctly causing a problem very similar to yours. When I removed the AAV and just had an open pipe where the AAV should sit it drained fine. Let me note that I dont like AAVs, but the can and do have their place.....when no other feasible option exists.

You mention that the pipe changes over to steel (could be cast iron, could be galvanized) and that the house is 104 years old. Knowing these facts would lead me to believe that you may have a flow restriction due to a clog, corrosion of the piping or a combination of both. Have you had this drain cleaned anytime lately?
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Old 10-23-2009, 07:52 AM   #7
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Sink gasping water


I snaked it when I removed the old fixtures and tub. It was clean. By the way, without the AAV the drain was very slow, now it just backs up about 1-2 inches before draining, no big deal. If the water is turned off after only an inch has accumulated, then the water drains out in about 10 seconds. Could there be another p trap farther down the line that I'm not seeing?
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Old 10-23-2009, 10:36 AM   #8
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With a house that old, there could very well be another trap. Back then, a "house trap" was commonly installed. Usually located just outside the foundation. Since then, it has been proven that the double trapping is unnecessary and causes problems. This is why double trapping of fixtures is against code. I have seen DIYers install a trap under a toilet, not realizing that there is an internal trap built into the toilet.

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