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Old 05-20-2019, 08:37 AM   #1
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Can I back flow from my tub to the grey tank ?

I want to minimize the height of the tub platform and am concerned about the instillation of my tub and getting the drain pipe sloped enough to the top of the grey tank.

So I was thinking that since my grey tank drain line goes right under the tub, that I could just plumb it into this pipe and back fill the grey tank.
( note that the dump valve is further down stream. )

I believe that with my research, that this should work just fine, but would like some reassurance from here. I figure that as long as the P trap or Hepv0 is above the top of the tank, that it should be fine.
https://www.vintagetrailersupply.com...-p/vts-017.htm

So, is this an option if I decide to do so ?
Thanks
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Old 05-20-2019, 08:41 AM   #2
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If your grey tank is very full, won't you have grey water backing up into the P-trap and possibly into the tub when you are moving, and the road action causes sloshing in the grey tank?



Also, you may lose the head of air just below the P-trap, thus allowing the odor of the grey water to fill the bathroom/trailer IMO. Plus . . . when you dump grey water, you may have grey water/odor come up above the P-trap as well.

You could probably live with all of the above possibilities, but "eyes wide open" is all . . .

Peter
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Old 05-20-2019, 08:46 AM   #3
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Yes, as long as the drain in your shower pan is higher than the top of the tank, you will drain the shower pan to the tank regardless of the path that the water takes.

But to the above point, when you move your trailer with a full grey tank, you may get sloshing up into the shower pan. A Hepvo or P-trap will keep odors and gases from coming up, but will likely not stop a slosh. Lots of people use an expansion plug in the shower drain to ensure that water isn't able to come up the drain and wet the shower pan. This might be the very simplest solution.

good luck!
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Old 05-20-2019, 08:59 AM   #4
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I would have the same concerns as OTR15 and Belegedhel. I would suggest using the hepvo valve not a P trap.

Also, install a valve (cable operated or electric so you can open/close from inside trailer) to keep grey water tank isolated when traveling. I did this between my two grey water tanks so contents wouldn't move between tanks when traveling. I used a cable operated valve. The cable must have gentle curves. The handle is inside my kitchen sink cab.
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Old 05-20-2019, 10:25 AM   #5
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Greyt ideas!



Tanks a lot!

Peter
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Old 05-20-2019, 12:50 PM   #6
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Unless I am missing something (and I could be), before much water goes to the gray tank, you will fill the drain line to your dump valve. After that, then yes, the water from the tub will backfill into the tank through the dump line. Your plan will work, but it really makes the trap or valve you use critical. I'm sure most of us have seen water back up into our shower while we washed dishes, etc. when our gray tank was full.
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Old 05-20-2019, 01:10 PM   #7
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Quote:
Originally Posted by fwjumper View Post
Unless I am missing something (and I could be), before much water goes to the gray tank, you will fill the drain line to your dump valve.
. . .


The line from the grey tank to the gate valve, at the exterior discharge location, already has grey water in it. Ditto for the black water behind its gate valve. When those tanks get full, the respective discharge lines are full of grey/black waste water.



Water finds its own level, whether it is in the horizontal waste line or the corresponding tank.

Tanks,

Peter
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Old 05-20-2019, 01:20 PM   #8
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Thanks for the comments and suggestions. keep them coming.

Now, I still do not see any difference in the design shown and one that pipes into the top of the tank. The tub is Above sub floor, the P Trap opening or Hepv0 will be above sub floor.
The tank is below sub floor.

If I piped into the top of the tank, then if I filled the tank, it is still going to back up into the tub also. Just as others have found out.

So, unless there is a physics reason such as more water pressure pushing, etc. The only difference would be in the extra 2 inches or such of tub platform height.

If moving with a full tank, then maybe the slosh in the wide area of the tank wound cause a fountain at the vertical pipe going up. That, I can understand.

I do like the comments and ideas and I do want to plumb into the top of the tank if I can. But, as is right now, I am thinking I would need 4 or 5 inch tub platform. verses a 2 or 3 inch. Head room is low to begin with.

I will do some mock ups to see what height I will need to get a slope on the drain line from the tub to the top of the tank. My tanks are flush up to the bottom of the sub floor, and with the screw in adapter and a 90 degree connection, the center line of the drain pipe into the tank is at 3.25 inch. so the tub drain pipe would have to be a little bit higher than that.

Looking forward to more comments.
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Old 05-20-2019, 02:36 PM   #9
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I agree with your thought process--to reiterate, whether you add water to the top of the tank, or to the bottom of the tank, as long as the surface of the water in the tank is lower than the surface of the water in the shower pan (even if by only the thickness of your subfloor), the pan will drain into the tank.

I have seen Airstream factory installations that ran drain pipes between the underside of the shower pan and the top surface of the subfloor, thus raising the shower pan at least a couple of inches. I assumed that it was done that way not for drainage, but because it was a mid-bath configuration, and the drain landed squarely on top of a frame rail, or over top of something like the fresh water tank.

I replumbed my trailer and did the shower-to-grey tank drainage essentially just like you plan to. One issue to be cautious of is that you make sure there is no trapped airspace in the top of your tank/tanks. I see in your diagram that you have the tank vented from the top, so you are already considering that.

good luck!

good luck!
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