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Old 07-23-2015, 09:30 PM   #1
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Shower drain acting as a vent for kitchen sink

My 31' 2013 classic when boondocking, after washing the dishes, you pull the drain the water drains from the sink quickly , venting into the shower, this happens even when the gray tank is empty. The vents are clean, I did check the one way air vents under the kitchen sink and under bathroom sink. If you plug the shower drain with a plug, the air will vent out of the bathroom sink. My 04 classic didn't act this way, I am thinking something might have a design problem ? Any ideas?
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Old 07-23-2015, 09:44 PM   #2
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Might need to run an eel down the drain line to clear any instructions in the drain pipe. Also remove the all the sink drain traps and clean them.

Could be a wasp nest at the top of the toilet vent pipe on the roof.
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Old 07-23-2015, 09:47 PM   #3
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The drain lines are clean, the sink drains into the holding tank via a T into the vent and the shower and bath sink go into the tank elsewhere, there are no blockages. Any ideas?
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Old 07-23-2015, 10:03 PM   #4
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My 75 AS has multiple vents. One for the kitchen sink, one for the shower and bath sink, and one for the toilet.

My guess is your kitchen sink vent is blocked.
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Old 07-23-2015, 10:05 PM   #5
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Hmmm, if the kitchen sink drains into the vent and you Have a full column of water entering the section of the vent pipe between a y in the vent stack and the tank, there is nowhere for the tank to vent past the water column during sink draining. That pressure has to go somewhere like the shower. Sounds like a design flaw to me.
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Old 07-23-2015, 10:06 PM   #6
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If your sink is using the shower to vent, then it sounds like your vent pipe is obstructed, but it sounds like you have already ensured that it is clear.

I have seen this issue described on the Forums before, and in some cases it is a design issue where the venting "capacity" isn't equal to the amount of water that is rushing down the drain. As I recall the situation, the vent pipe comes off of the pipe that is draining the sink into the tank. If the pipe is full of water, then the air that needs to be displaced from the tank cannot go up the proper vent, and instead heads for the shower trap.

To confrim that it is design problem, you might try pouring water down the sink slowly and seeing if it vents from the shower still.

good luck!
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Old 07-23-2015, 10:06 PM   #7
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It sounds like the vent stack in restricted between the T and outside. Can you look down the vent from the outside?
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Old 07-23-2015, 10:11 PM   #8
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I'm pretty sure I have a dedicated stack for black and gray combined. The gray has 2 openings in addition to the vent. One for the shower and kitchen combined. And one for the bath sink. I have a 30 classic with split bath
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Old 07-23-2015, 10:22 PM   #9
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Tomorrow I will pour a bucket of water slowly down the kitchen sink drain and see what it does, never noticed this before, but it doesn't seem to do it when the drain is open.
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Old 07-23-2015, 10:23 PM   #10
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Shower drain acting as a vent for kitchen sink

My gray tank is curb side rear of trailer. My kitchen sink vent is above the kitchen curb side center. My rear bath shower (curb side) and sink (center) is vented above the shower. The black tank is under the toilet street side rear and vented above the toilet.

I'd pour the water down the vents.
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Old 07-23-2015, 10:29 PM   #11
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Quote:
Originally Posted by tjdonahoe View Post
Tomorrow I will pour a bucket of water slowly down the kitchen sink drain and see what it does, never noticed this before, but it doesn't seem to do it when the drain is open.
When the sink faucet is just running or a bucket is poured slowly, you don't have a full column in the pipe and air can escape past the draining water.
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Old 07-23-2015, 11:05 PM   #12
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I can tell you exactly what is going on. From your description in post 3, the drain from the sink T's into the vent stack, vent goes straight up, drain enters the t from the side. What is happening, is that when water is flowing quickly down that pipe and into the tank it is hitting the T and flowing down, this blocks the air from the tank from being able to flow back up the same pipe that is also the vent. This causes slight pressure to build up in the tank which works its way out by path of least resistance which in your case is the shower drain.
Seen this before in RV;s and houses. My bath sink is plumbed the same way in my 79 safari.
Several ways to fix this, the easiest being drain the sink slowly. This keeps the water from rushing and filling the pipe at the vent T and allowing air to escape.
The others would require some re-plumbing. Can you see the top of the tank? Are there any unused connection ports that you could route the drain or vent to? You could also try to put hepvo traps in the line for the shower drain, replacing the p-trap, but that would then likely try to push air back up another route, so you'd have to put them on all the other gray water drains as well. Does the T where the drain and vent merge have a 90 degree T or does it have some sweep/arch from the side drain inlet?
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Old 07-24-2015, 01:11 AM   #13
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My 2008 27FB and others with the large round sink find that with the shower drain unplugged the dishwater actually comes up in the shower a bit if you drain the sink fast. It's a known flaw and if the sink water cuts off the vent on it's way to the gray tank just over the shower it explains why. I'm over it and rinse the shower if it happens. As we boondock some we usually use a collapsable washtub and dish rack for washing dishes and dump the dishwater in the toilet and leave the gray water tank to hold shower and hand washing water as gray always fills before black unless there's something terribly wrong. - Brad


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Old 07-24-2015, 09:54 AM   #14
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I filled the kitchen sink and let it drain out SLOWLY ,it did not vent into the shower. The kitchen sink that drains into the vent pipe is connected with a straight T. I am going to call the factory and tell them about this situation....
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