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Old 10-23-2019, 12:03 PM   #1
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Is my grey water venting / ptrap logic sound?

Please critique this plan & let me know what you think. I'm not much of a plumbing wiz:

Our 2 sole indoor water fixtures will be a shower and a kitchen sink, located side-by-side divided by a common wall. Each will tie into a common drain pipe, which will run ~ 10' to our grey water tank.

From what I think I understand, hepvo valves ('waterless' ptraps) don't require fixture venting. So would it work to tee the sink & shower drains into the common drain pipe, put a single hepvo valve on that (under the bus below both fixtures), and then run that to the grey water tank? The tank itself would be vented to the roof using the same ID as the incoming drain pipe.

If this doesn't make sense, let me know, & I'll supply scribbles.

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Old 10-23-2019, 12:46 PM   #2
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Something like this?
Redundancy using both. The Hepvo keeps the stink contained in the tank. If it's vented out the roof, the hepvo isn't needed.
I need to swap the 2 tanks, as the forward fresh tank is shorter than the gray tank. But I can get proper slope with the larger tank in back.
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Old 10-23-2019, 01:17 PM   #3
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Yep. Exactly what that.


Not sure what you mean by 'redundancy using both', though, Marc. Could you elaborate? Are you saying both a tank vent and the hepvo valve? Or a hepvo valve, tank vent, AND another vent near the fixtures?
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Old 10-23-2019, 01:21 PM   #4
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Quote:
Originally Posted by o1marc View Post
Something like this?
Redundancy using both. The Hepvo keeps the stink contained in the tank. If it's vented out the roof, the hepvo isn't needed.
I need to swap the 2 tanks, as the forward fresh tank is shorter than the gray tank. But I can get proper slope with the larger tank in back.
Even with venting through the roof, you want a P trap or hepvo. Without I you will get tank stink in your living space.

All of your tanks should be vented. The displaced air, when you add fluid, needs someplace to go. Without a vent you will likely have drains that flow slowly and a freshwater tank that takes a long time to fill.
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Old 10-23-2019, 01:33 PM   #5
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Originally Posted by TheHubbardBus View Post
Please critique this plan & let me know what you think. I'm not much of a plumbing wiz:

Our 2 sole indoor water fixtures will be a shower and a kitchen sink, located side-by-side divided by a common wall. Each will tie into a common drain pipe, which will run ~ 10' to our grey water tank.

From what I think I understand, hepvo valves ('waterless' ptraps) don't require fixture venting. So would it work to tee the sink & shower drains into the common drain pipe, put a single hepvo valve on that (under the bus below both fixtures), and then run that to the grey water tank? The tank itself would be vented to the roof using the same ID as the incoming drain pipe.

If this doesn't make sense, let me know, & I'll supply scribbles.
Are you using the waterless trap due to space constraints?

Can you run 2 waste pipes and tee just at the tank? The reason I bring this up is that if there is a restriction in the pipe there’s a chance of the kitchen water coming up in the shower. 2 of those waterless traps could help prevent that to some degree since it is acting also as a backflow device more than a p-trap would. If you use the waterless trap, make sure it’s accessible for service. You aren’t going to be able to snake it.

For a drain to work it needs air to relieve the suction of the water running down the pipe. In the setup you are describing, the drain is the vent. 10’ is a long wet vent. I’m not saying it won’t work. The volume of waste water we are working with is small. It’s just something to keep in mind when designing the system.
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Old 10-23-2019, 02:01 PM   #6
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Are you using the waterless trap due to space constraints?

Can you run 2 waste pipes and tee just at the tank? The reason I bring this up is that if there is a restriction in the pipe there’s a chance of the kitchen water coming up in the shower. 2 of those waterless traps could help prevent that to some degree since it is acting also as a backflow device more than a p-trap would. If you use the waterless trap, make sure it’s accessible for service. You aren’t going to be able to snake it.

For a drain to work it needs air to relieve the suction of the water running down the pipe. In the setup you are describing, the drain is the vent. 10’ is a long wet vent. I’m not saying it won’t work. The volume of waste water we are working with is small. It’s just something to keep in mind when designing the system.
Understood on all counts. No, running two pipes into the grey water tank is NOT doable. I barely have enough room for one (hence my other thread that you've been a great help on).

How about if I add an AAC vent under the sink, in-between the sink & shower?

Good point about the waterless trap & snaking. It will, fortunately, be exposed, directly below the bus flooring. And yes, it was chosen based on space/routing constraints.

The thing is, on this short bus, with air brakes, AC condensors, & everything else either already there or that I plan on adding, there's literally one path I can use to route the greywater drain without encountering significant difficulty, as well as only one place the tank can go.
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Old 10-23-2019, 02:48 PM   #7
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Originally Posted by TheHubbardBus View Post
Understood on all counts. No, running two pipes into the grey water tank is NOT doable. I barely have enough room for one (hence my other thread that you've been a great help on).

How about if I add an AAC vent under the sink, in-between the sink & shower?

Good point about the waterless trap & snaking. It will, fortunately, be exposed, directly below the bus flooring. And yes, it was chosen based on space/routing constraints.

The thing is, on this short bus, with air brakes, AC condensors, & everything else either already there or that I plan on adding, there's literally one path I can use to route the greywater drain without encountering significant difficulty, as well as only one place the tank can go.
I know all about space limitations. I’ve been searching parts and designing/redesigning for several weeks now. Im glad I waited. I’m definitely playing Tetris like a professional contortionist.

How much drop do you have in that 10 feet?

Can you fit a waterless trap for each drain?
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Old 10-23-2019, 02:58 PM   #8
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Originally Posted by TheHubbardBus View Post
Yep. Exactly what that.


Not sure what you mean by 'redundancy using both', though, Marc. Could you elaborate? Are you saying both a tank vent and the hepvo valve? Or a hepvo valve, tank vent, AND another vent near the fixtures?
No need for a Hepvo if the tank is vented out the roof. I don't have roof vent. The tank contains its smell until I drain it. Hepvo prevent back venting.
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Old 10-23-2019, 03:04 PM   #9
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No need for a Hepvo if the tank is vented out the roof. I don't have roof vent. The tank contains its smell until I drain it. Hepvo prevent back venting.
You haven’t vented the tank? I’m curious about tank pressure caused by temperature change and microbes.
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Old 10-23-2019, 03:24 PM   #10
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Originally Posted by TheHubbardBus View Post
Understood on all counts. No, running two pipes into the grey water tank is NOT doable. I barely have enough room for one (hence my other thread that you've been a great help on).

How about if I add an AAC vent under the sink, in-between the sink & shower?

Good point about the waterless trap & snaking. It will, fortunately, be exposed, directly below the bus flooring. And yes, it was chosen based on space/routing constraints.
The ideal thing would be one Hepvo valve in the down pipe under the sink and another under the floor for the shower. Combine the two pipes downstream of the Hepvo valves with a sanitary wye or tee, then run that one pipe to the gray tank.

I really like the idea of having a Hepvo as near to the shower drain as possible because of backflow. As Danjo mentioned, kitchen water could come up at the shower. Another possibility arises if the bus is parked sloped the wrong way. The drains might work but leave some water standing in the pipe between the shower and the single Hepvo. That water could come sloshing up later when the bus is moved.

You also asked about an air admittance valve (AAV). Their sole purpose is to let air into a drain system. They can resolve a situation where the water is sometimes sucked out of a P trap. Since you don't have P traps in your plans an AAV isn't useful for you.
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Old 10-23-2019, 03:29 PM   #11
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You haven’t vented the tank? I’m curious about tank pressure caused by temperature change and microbes.
Good point. I may install a studor vent. Can vent it straight off the top of the tank or put a 90 on it and send it out the back wall of the storage bin.
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Old 10-23-2019, 03:33 PM   #12
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Originally Posted by PNW_Steve View Post
Even with venting through the roof, you want a P trap or hepvo. Without I you will get tank stink in your living space.

All of your tanks should be vented. The displaced air, when you add fluid, needs someplace to go. Without a vent you will likely have drains that flow slowly and a freshwater tank that takes a long time to fill.


I have 2 fresh tanks combined by a connection hose. Can't vent the bottom tank as it is full half the time. Fill spout has a vent for fast filling.
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Old 10-23-2019, 03:44 PM   #13
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The ideal thing would be one Hepvo valve in the down pipe under the sink and another under the floor for the shower. Combine the two pipes downstream of the Hepvo valves with a sanitary wye or tee, then run that one pipe to the gray tank.

I really like the idea of having a Hepvo as near to the shower drain as possible because of backflow. As Danjo mentioned, kitchen water could come up at the shower. Another possibility arises if the bus is parked sloped the wrong way. The drains might work but leave some water standing in the pipe between the shower and the single Hepvo. That water could come sloshing up later when the bus is moved.

You also asked about an air admittance valve (AAV). Their sole purpose is to let air into a drain system. They can resolve a situation where the water is sometimes sucked out of a P trap. Since you don't have P traps in your plans an AAV isn't useful for you.

Excellent. That's totally doable. Plus, between you & Danjo, I now understand the 'why' behind the 'do'. Many thanks!!
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Old 10-23-2019, 03:53 PM   #14
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I know all about space limitations. I’ve been searching parts and designing/redesigning for several weeks now. Im glad I waited. I’m definitely playing Tetris like a professional contortionist.

How much drop do you have in that 10 feet?

Can you fit a waterless trap for each drain?

As per above, yep, I can and will put one hepvo off each fixture. I can locate each pretty much right off the respective drain.

Drop... I've just eyeballed it so far, but it looks like around 2" before I start running into obstructions. That's assuming the bus is flat. Right now it's 'stink-bugging' from lack of weight, magnifying the issue. Hopefully the build takes care of that, but I have no way of knowing how much the suspension will sag.
LOL on the contortionist bit. Know exactly where you're coming from.
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Old 10-23-2019, 04:31 PM   #15
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You haven’t vented the tank? I’m curious about tank pressure caused by temperature change and microbes.
We didn't put a vent on our grey tank, but it's "vented" by virtue of the fact that the 1 1/4" house drain dumps into a 1 1/2" hole in the top of the tank. There's a bit of a gap around the drain pipe where it penetrates the tank. This seems to be enough venting to alleviate any odors wafting into our living space (we have P traps of course) and there's no back pressure. The only issues we've encountered are that there's some splashing when we move, so our luggage compartment where the tank is located gets moist, and when we drain the tank the P-traps get emptied and the bus gets some stank. I've been meaning to add a vent in the top of the tank and seal the drain penetration with a gasket, but aside from those 2 issues there haven't been any problems.
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