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Discussion Starter · #1 ·
I'm planning a water system with a UDDT porta-potty (not plumbed) but a sink and shower. I want the sink and shower to drain into a gray tank mounted underneath, but there's a frame/chassis bar between my drain spot and my tank. I'll use a Hepvo 1-way valve slanted down, just under the floor after a "T" where the drain lines meet. My questions:

1.The drain will go through the floor, through a slanted Hepvo, then pipe will have to make a "U" shape to go under a frame piece and into the tank. The entire tank will still be beneath the level of the Hepvo filter valve, but much of the other downstream pipe will be lower than a large amount of the tank. Will the entire capacity of the tank get used properly (filling up along with the final section of pipe? Somehow I'm having trouble imagining the physics or thinking I'm forgetting something.

2. In residential plumbing, all pipes are connected to vents through a building's ceiling, in order for pressure to equalize as water moves in and out of pipes. This doesn't ever seem to be duplicates in campervans - why not? Will this design avoid "gurgle" or draining problems, since i have a hole in the top of the gray tank?

3. Anyone w/a similar design use electrically switch heat tape on the pipe below the van to prevent a freezing ice dam on ski trips?

THANK YOU


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2021 3500 extended in Michigan
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Water always tries to find it's own level so as long as your Hepvo trap is above the top of the tank you should be fine. My grey tank setup is similar to yours but my tank inlet is lower, just below the frame rail and my Hepvo is above the floor in the sink cabinet.

Normal residential plumbing uses a water filled P trap to keep sewage gasses from backing up into the room. The roof plumbing vents stop drain water from siphoning the trap. The Hepvo traps don't use water to seal out sewage gasses so no roof venting is necessary. When water drains through the Hepvo it sucks air directly from the sink drain so as long as the grey tank itself is vented there is no gurgling.

RV drains that use P traps usually use a "cheader vent" in place of a through-the-ceiling vent. The cheader vent pulls air from under the counter but is a one way valve and doesn't allow sewer gasses to back up into the RV.
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I have a similar system with Hepvo valves for the sink and shower. Both Hepvos are above the top of the tank and I have an always-open vent/overflow on top of the tank. I can verify the vent/overflow works to keep grey water from backing up into the Hepvos (DAMHIK). The drain runs are short and I've never regretted not having a stack vent or cheater vents. The tank vent works just fine for that, too.

My only concern about your U-loop under the crossmember would be trapping water that could freeze. Maybe enter the tank lower and eliminate the trap.

Fun fact: Some folks connect their inflow at the bottom of the tank, teed in with the tank drain. Also not the best for freeze protection, but the physics still works. It just backs up into the drain pipes immediately. A tank vent/overflow prevents it from backing up too far. No harm, no foul.
 

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Most water tanks will have threaded inlet/outlet fittings molded into the tank. If you want a custom vent location you can spin weld fittings anywhere you want but it can be a hot, nasty process. Bulkhead fittings are another option. For my tank vent I just threaded an pex "L" into my upper tank fitting and attached a piece of pex pipe with some window screen clamped to the end to keep out bugs. No need to get too fancy, you just need to get air in and out when filling and draining.
 

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Vanguy, looks like you will need heat tape on the Pex inlet line, but how are you keeping the grey water tank from freezing? Running heat tape and a tank heating pad will suck up a lot of amps.

Is there any way to keep the Pex above the floor until it can drop straight down into the grey tank?
 

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Where do you place hole in tank (as vent and overflow) to prevent losing water before it is full?
My vent is on top of the tank, but high on the side with a vertical elbow works too. It doesn't have to be big. 1/2" is plenty of air, but I used 1-1/2" ABS pipe and a rubber tank grommet I had on hand. I used those grommets in all my DIY tank holes.
 

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My vent is on top of the tank, but high on the side with a vertical elbow works too. It doesn't have to be big. 1/2" is plenty of air, but I used 1-1/2" ABS pipe and a rubber tank grommet I had on hand. I used those grommets in all my DIY tank holes.
I was thinking more about how much water will overflow prior to tank being full every time driver brakes, accelerates, or goes around curves, depending on where the hole was drilled on top of tank. Unless overflow is significantly higher over tank, water will spill out when it “flows up”, just due to acceleration.

A van is not stationary like a house. If you find you don’t have to empty too often, maybe it already did a partial discharge. 😀
 

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I was thinking more about how much water will overflow prior to tank being full every time driver brakes, accelerates, or goes around curves, depending on where the hole was drilled on top of tank. Unless overflow is significantly higher over tank, water will spill out when it “flows up”, just due to acceleration.

A van is not stationary like a house. If you find you don’t have to empty too often, maybe it already did a partial discharge. 😀
I see your point. We normally dump often enough that it almost never gets that full. Placing the vent dead-centre on top should help minimize (not eliminate) slosh spillage. Plus, our grey water is just soapy water. It has never gone "off" before we dumped it, so I'm not too concerned about the occasional road spillage.
 

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The hepvo valve is probably un-necessary and redundant as it appears from your design you are putting a p trap before the tank. Just a thought. It won't hurt anything and can be a backup in case the trap runs dry.
 

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The hepvo valve is probably un-necessary and redundant as it appears from your design you are putting a p trap before the tank. Just a thought. It won't hurt anything and can be a backup in case the trap runs dry.
If I understand correctly that "p-trap" was not intended as a p-trap, it's just a detour around a frame member. I cautioned about it's freeze potential with no way to drain it. It would be better to enter the tank lower and eliminate the unintended p-trap. That way simply draining the tank would winterize the drains too. Otherwise, you may need a drain-**** or heat cable in that "p-trap". Hepvos are frostproof if installed correctly.
 

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If I understand correctly that "p-trap" was not intended as a p-trap, it's just a detour around a frame member. I cautioned about it's freeze potential with no way to drain it. It would be better to enter the tank lower and eliminate the unintended p-trap. That way simply draining the tank would winterize the drains too. Otherwise, you may need a drain-**** or heat cable in that "p-trap". Hepvos are frostproof if installed correctly.
I believe this is the most logical answer - you'll never drain the "unintended P" without a siphon.
 

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If I understand correctly that "p-trap" was not intended as a p-trap, it's just a detour around a frame member. I cautioned about it's freeze potential with no way to drain it. It would be better to enter the tank lower and eliminate the unintended p-trap. That way simply draining the tank would winterize the drains too. Otherwise, you may need a drain-**** or heat cable in that "p-trap". Hepvos are frostproof if installed correctly.
I agree, it wasn't put there for that, but it is one. And it definitely can freeze, and quickly being the lowest point, exposed to the wind!

I just pour a little non-toxic rv antifreeze in mine, but my drain line is pretty small and my p trap down there is intentional
 

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Discussion Starter · #18 ·
Thanks everyone for the thoughts.

I do plan to make an unintended "P trap" below that part of the frame, but I can't avoid it with where I want my grey tank and floor drain.

I guess that means I don't need to bother with a Hepvo P-trap replacement though, and I can just use 1" reinforced flexible tubing!

I guess I'll buy a few feet of the 12v heat wire that @Hein suggested and snake it through the tubing, or else this alternate:


It seems like if I'm actually going to rely on that low dip as the functional "p-trap" then I shouldn't want to have any drain valve there.

Anyone have a favorite simple electrical on-off valve for a 1/2" or 3/4" pipe?
 

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I guess that means I don't need to bother with a Hepvo P-trap replacement though, and I can just use 1" reinforced flexible tubing!
It seems like if I'm actually going to rely on that low dip as the functional "p-trap" then I shouldn't want to have any drain valve there.
If you rely on a flex-pipe for freeze protection, then a p-trap drain valve would be redundant. To be safe, the flex-pipe should probably start (at least) level with the top of the tank, since that would be the water level in the pipe when the tank is full.
 
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