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Old 01-20-2014, 01:19 PM   #1
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does this make sense? shower pan/tank connection

In order for the grey tank to fit we have to put it in the bay just behind the shower pan and have to connect the pan with a pipe around the frame.
We're hoping to avoid drilling through the cross member, would the attached mock up work for water flow?
My thought process is since the grey tank is below the shower pan the water will flow into the tank before the pan.

Would this ever work?
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Old 01-20-2014, 01:41 PM   #2
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Just a thought - It probably will work like a 'P' trap under a sink. Good & Bad; Good: that it will probably keep a 1/2 full tank of gray water from coming up into the shower when you travel on a curvy road - which mine does (can't use the shower for storage when traveling). Bad: that it might slow down (the drop is not that great) the flow of water out of the shower.
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Old 01-20-2014, 01:49 PM   #3
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that was my thought, on the 'p' trap function
will there ever be a reason water wouldn't flow into the grey tank (in terms of gravity/pressure)
would it be smart to invest in a fitting that only allows water flow in one direction?
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Old 01-20-2014, 01:55 PM   #4
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the hepvo (HepvO Self Sealing Waterless Waste Valve) will prevent water coming back up into the shower pan.
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Old 01-20-2014, 02:00 PM   #5
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mock up!

just realized my mock up didn't attach!

Click image for larger version

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Old 01-20-2014, 02:07 PM   #6
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I think your problem is going to be related to the facts that water has weight, and the law of gravity . . . I've got a friend who used one of those marine shower sump pumps and is happy with it (all things considered).
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Old 01-20-2014, 06:26 PM   #7
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If its vented shouldn't the water level out and seek lowest point? Why would it NOT work?
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Old 01-20-2014, 06:36 PM   #8
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the weight/volume/mass of the water in the tank will create a certain amount of pressure on the contents of that pipe. If you have a "U" shaped hose and you put water in it, it'll equal out / level on each side of the "U". If you apply pressure to one side it's going to come out the other. It can be air pressure or otherwise. If the tank didn't have sides then it would work fine . . . OR if the shower were something like 10 feet above the tank it might also work . . .
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Old 01-20-2014, 07:33 PM   #9
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Quote:
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the weight/volume/mass of the water in the tank will create a certain amount of pressure on the contents of that pipe. If you have a "U" shaped hose and you put water in it, it'll equal out / level on each side of the "U". If you apply pressure to one side it's going to come out the other. It can be air pressure or otherwise. If the tank didn't have sides then it would work fine . . . OR if the shower were something like 10 feet above the tank it might also work . . .
Fluids seek their own level. The water level in the pipe will equal the water level in the tank. That means that the hydraulic head that drives the drainage is equal to the difference in elevation between the water level in the shower pan and the level in the tank. In other words, mere inches. Your shower will drain slowly. VERY slooooooowly. A check valve to prevent backflow will make it drain even more slowly.
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Old 01-20-2014, 08:01 PM   #10
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my tiny brain still wants to believe there's "head" pressure on the contents of the pipe that would push it out . . . i guess there are a couple brain cells that kind of get it. sorry for acting like i knew what i was talking about . . . kind of embarrassing.

having said that, i still wouldn't do it . . .
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Old 01-20-2014, 08:32 PM   #11
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my tiny brain still wants to believe there's "head" pressure on the contents of the pipe that would push it out . . . i guess there are a couple brain cells that kind of get it. sorry for acting like i knew what i was talking about . . . kind of embarrassing.
No apologies needed, okay? Hydraulics are complicated; that's one reason why engineers pay so much to get a college degree. That and college is still one of the best places to pick up smart women…

"Pressure on the contents of the pipe would push it out…" only works if you don't have water pressure in the holding tank pushing back. Hydraulic head is the difference in pressure from one end of the pipe to the other. You can trick gravity to some extent by having a siphon, but for that you would need a P-trap. By the way, a siphon is exactly how a home toilet flushes; add water in the bowl until it's higher than the siphon built into the base, and then sploosh, the water is sucked out. But with just a U-shaped pipe— or in this case I guess it's a J-shaped pipe since one end is higher than the other— there's no siphon effect, just gravity flow.
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Old 01-20-2014, 09:03 PM   #12
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"Pressure on the contents of the pipe would push it out…" only works if you don't have water pressure in the holding tank pushing back.
I guess I was "thinking" the opposite . . . that water added to the shower pan can't overcome the pressure created by a half filled holding tank . . . that somehow that holding tank body of water would pressurize the line and want to exit into the shower pan simply because it had more "mass" . . . which i'm guessing is still not true.

(and thank goodness for the siphon effect)
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Old 01-21-2014, 08:46 AM   #13
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the alternative is still gravity based and will likely still drain the shower pan slowly. the alternative being, cut a hole in the crossmember and drain shower into the grey tank without going below the frame. They are trying to avoid cutting into the cross member.

Which is preferable?
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Old 01-21-2014, 09:00 AM   #14
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Why not raise the shower pan about 2 1/2"? Then install a "P" trap below it. Have the discharge of the "P" trap go over the cross member into the top of the GW tank with a 90 degree elbow.
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