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Old 03-13-2015, 05:56 PM   #1
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Black tank overflow

I have a 2007 George Town where is the over flow for the black tank? Will it come out of the toilet or will it come out of the vent on my roof first?

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Old 03-13-2015, 06:13 PM   #2
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There is no designated "overflow" for the black or grey tank. The gray tank over capacity most likely would show up in your shower (lowest point) and the black tank most likely in your toilet first but the vent tube is smaller than the toilet opening so not sure. Think maybe the toilet. Liquids seek their own level.

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Old 03-13-2015, 06:20 PM   #3
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Originally Posted by eswan21 View Post
I have a 2007 George Town where is the over flow for the black tank? Will it come out of the toilet or will it come out of the vent on my roof first?
Well, fortunately I have never experienced an overflow, but I would have to go out on a limb here and say the principles of physics would have to apply. I see no way a toilet system that operates on gravity principles could force water, and other substances, into a tank, create pressure and force liquids or solids several feet up through a vent stack and out the vent on the roof.

Just like in a house, septic tank gets full, toilet overflows. Never saw waste coming out of a vent stack on a home roof. Someone else may have, but thankfully I have not.

My outdoor kitchen sink drains into my black tank and I still have to assume as it drains from gravity that even if the black tank filled, the toilet stayed closed and I kept running water into the sink that the sink would just overflow and nothing could be or would be forced up the vent pipe.

Maybe someone else will share an experience otherwise?
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Old 03-14-2015, 01:04 AM   #4
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It would come up thru the toilet and make a hell of a mess. Not had that problem and hope I never will.
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Old 03-14-2015, 09:15 AM   #5
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I've intentionally filled the black tank to the point that you can see it in the pipe below the toilet valve...also makes a different sound when flushed. That way I have an idea of capacity in terms of days use instead of relying on the inaccurate level monitor.

Most 5th wheels don't have a straight drop into the black tank but offset using 45 elbows. Most TT's would be a straight drop into the tank so you can verify the liquid by looking down the hole with a flashlight...don't drop the flashlight!


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Old 03-14-2015, 09:40 AM   #6
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Luckily my unit also has a straight shot into the tank. It does allow for a quick visual of the waste level.

I would have to agree that the waste would come out of the toilet first before ever coming out the roof pipe.


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Old 03-15-2015, 12:43 PM   #7
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Eswan, your question has two different ways of looking at it.
I have thought the same question. It depends if one is just flushing the toilet and if the black water tank is full then the toilet would not drain. If one is flushing the black water tank with the drain valve closed then the water would probably come out the vent. Given the toilet valve is water tight, which it is designed to be. Though there is an O-ring in the toilet itself that could be weak given age, it then could leak there.
I have no personal experience at this so it is just all a guess. I never leave the drain valve closed when flushing the blank water tank.
You posted a good question.

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Old 03-15-2015, 01:17 PM   #8
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I'm not a plumber, but here would be my take on what would happen...

Your system is not pressurized. If your black tank was full the level in your vent (at best) could be level with the toilet... If you continued to "add" to the fullness of the tank, you will back up to your toilet and see the condition, by not being able to flush - like a backed up toilet at home.... If the water valve got stuck with the drain partially open and you weren't around it would fill to the top of the toilet, including the vent and then onto the floor.. I don't know that anything would get pushed up the vent and I highly doubt you'd see any overflow on your roof...If you did, your trailer would be "really full" and pretty much a write off..

This is all guessing though...
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Old 03-15-2015, 01:22 PM   #9
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I went out with my father about three years ago (GRHS). I had my trailer and he brought his class A. He accidentally hooked the fresh water hose to the black tank flush and turned it on. It definitely comes up out of the toilet! My stepmother was non to happy!!!

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