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Old 11-13-2022, 01:45 PM   #1
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Venting my gray tank

Iím planning on having a 40 gallon gray tank in my skoolie. Only a kitchen sink, a bathroom sink and a shower will drain into it. No urine. The drains will have Hepvo valves, so no P traps that might lose their water and allow smells into the bus.

From research in this forum and elsewhere, I assume I need to vent the gray tank. When wastewater flows into it, presumably (smelly) air in the tank will be displaced that has to escape somewhere.

I assume that the vent must be higher than any fixtures that drain into it. Do I want to vent the tank as high up as possible so the smell does not blow back into an open window? That pretty well means drilling a hole in the roof that Iíll need to make weathertight. Thoughts on placement?

Those of you who have done such venting, Iíd appreciate details. What diameter pipe/tubing did you use? Is 1 ľ inch enough? What kind of vent did you mount in the hole you drilled in the roof, that lets gas out but doesnít let rain or critters in?

Thanks.

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Old 11-13-2022, 02:47 PM   #2
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Hi David
Yes the vent goes thru the roof on an rv tank similar to a house vent. The higher the better when venting, (think of a fireplace chimmney creating draft) the attached pic is the standard rv vent cap, your 1.25 abs would go thru the roof and into this cap then sealed with dicor self levelling sealant.
FYI your kitchen sink is considered black water as kitchen scraps contain proteins and will cause much more stink in your tank so the tank needs treatment for that.
Safe travels
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Old 11-13-2022, 07:39 PM   #3
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Hi David, I am venting a 70 gal grey water tank (sink and urine) with a 3/4” hose and then a 3/4” horizontal pipe at the top of the vertical side wall, just before the roof curvature. The pipe only protubes by 1/4”. Has been working well so far. Using a 3/4” hose made it easy to route in the wall up.
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Old 11-13-2022, 10:41 PM   #4
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You both have been super helpful. Thanks. I just ordered that cap.
Nice to know even a 3/4 inch hose us adequate.
Is there a treatment you recommend for stinky food waste?
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Old 11-14-2022, 08:03 AM   #5
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I used a cap similar to the photo above. I didn't want to put a hole in my roof so mine vents out the back. I did modify the base a little by creating a small groove in the base to prevent water from accumulating.
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Old 11-14-2022, 10:52 AM   #6
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Thanks for the description and the pix!
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Old 11-15-2022, 01:33 AM   #7
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And here's what I did:
https://www.skoolie.net/forums/f50/d...tml#post472637

One reason I did it this way was to avoid any roof penetrations: I have 2kW of solar panels up there, so vents wouldn't work too well of they're under the panels. Obviously the main reason to faff about with double vent pipes per tank is to assure an adequate through-flow of fresh air in each waste tank. I've never heard of anyone else doing it this way, but that's typical of much of my conversion. Don't be afraid of doing things differently!

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Old 11-15-2022, 09:26 AM   #8
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I am planning on venting my gray tank into my composting toilet enclosure where any stink will be pulled up a powered vent to the roof.

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Old 11-15-2022, 02:57 PM   #9
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I would not think you'd need to vent any higher than a couple inches over the tank since it's not a pressurized system? I'd just keep the vent under the bus and away from windows...
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Old 11-15-2022, 05:51 PM   #10
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FYI your kitchen sink is considered black water as kitchen scraps contain proteins and will cause much more stink in your tank so the tank needs treatment for that.
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I've used various recreational systems over the decades ranging from 1/2 a tent and 1/2 the poles and lines with the other half carried by another Marine, though slide in campers, bumper pulls, 5th wheels, and motorhomes. NEVER used any sort of treatment for the GRAY tank and never heard of the concept of kitchen waste being "black" until the last few years. Black water contains fecal matter.
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Old 11-15-2022, 05:53 PM   #11
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I would not think you'd need to vent any higher than a couple inches over the tank since it's not a pressurized system? I'd just keep the vent under the bus and away from windows...
The reason for venting higher than all fixtures is that a vent below a fixture allows a full tank to then drain out the vent. Mounting the vent higher than all fixtures means that when the tank is full, your sinks will back up rather than (in most places) unlawfully dumping through the vent tube.
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Old 11-15-2022, 06:02 PM   #12
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I am planning on venting my gray tank into my composting toilet enclosure where any stink will be pulled up a powered vent to the roof.
Ted
This will vent your gray tank but will not prevent a full tank from then "venting" liquid directly into your poo bucket.
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Old 11-15-2022, 06:22 PM   #13
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Originally Posted by HamSkoolie View Post
The reason for venting higher than all fixtures is that a vent below a fixture allows a full tank to then drain out the vent. Mounting the vent higher than all fixtures means that when the tank is full, your sinks will back up rather than (in most places) unlawfully dumping through the vent tube.
That makes sense. Thanks for the clarification!
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Old 11-15-2022, 09:28 PM   #14
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British Columbia government waste water definition
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Old 11-16-2022, 07:11 AM   #15
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According to this document from BC, to be black water it must contain feces. But gray water does not include water from a kitchen sink. So water from a kitchen sink appears to be neither gray water nor black water.
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Old 11-16-2022, 01:13 PM   #16
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both of my waste water tanks are tied into one vent, which is mounted high on the bus.
Why are waste tank vents mounted high? Picture a hot, summer day, no breeze, and you want to relax outside under the shade of your awning. Take a deep breath, yuk...you can almost taste the smell.
Is a single vent adequate? I argue that it is, the vent allows water to enter and exit the tank. Water entering the tank is not going to be a significant amount, all at once. Even the toilet is only going to ad about a gallon at a time. And it's highly unlikely that you would be running sinks, toilet and shower water simultaneously.
By far, the biggest change in tank water level is when you empty the tanks. When you do, it's black water first, then the gray. Not at the same time, the gray water rinses out the sewer hose, etc which reduces the smell and makes it a least a little more sanitary (not completely).
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Old 11-21-2022, 09:32 PM   #17
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FYI, I have installed the gray tank vent. It's not yet hooked up to the tank because I haven't yet bought or installed the tank. But I drilled a hole in the side of the bus above the windows and completed that end of the installation. I've posted a YouTube video about the it:
https://youtu.be/ji3IfM11x-s
(Starts at 23:11)

Apart from a recomendation about the vent cover (thanks, Oscar1) I was not able to find much online about how manage the part where it exits the bus. So I pretty well winged it. If any part of my approach makes you smile (or makes you wince) feel free to share your thoughts so we can all learn.
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Old 11-22-2022, 12:16 AM   #18
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Well done video.
Maybe you should create a path of escape for any water that may accumulate under the gray water tank vent.

My only other question is how all this will fit under whatever interior ceiling/wall covering you will use. Between the barbed fitting and the plastic vent tubing, you may need quite a bit of space. Bend that tubing too much and it will kink.

Only thing that I can think of at this moment would be to get a 1" heater hose with a 90 molded into it. You will still need space, but the chance of kinking the vent hose is reduced.
https://i.ebayimg.com/images/g/M2YAA...Ugk/s-l400.jpg
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Old 11-22-2022, 01:00 AM   #19
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i've used 1/2" pex as vent tubing on my bus, and marine fuel tank vents as the exit vent for the tanks. look for a vent that has an elbow on it.

i did not want to put a hole in the roof. i placed the vent high in the driver's side wall away from windows.

12 years since the first one and all works fine. did the same thing with a van last year and used 5/8" clear tubing. easy peezy installs.... works great
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Old 11-22-2022, 07:22 AM   #20
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My thoughts about hiding it aren't yet fully formed, but the tentative plan is to hide it in a mirrored vanity cabinet. I like the heater hose idea. Thanks.
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