Tank Venting???

[TPdrums]

I am rebuilding the interior of my Overlander. It started out as a Rear Bath, and then the PO remodeled to a custom mid bath, with a seperate shower. Now I am rebuilding to a mid ship wet bath. The black water vent is in a really bad and obtrusive spot, I want to move it. A good option is to join it with the grey vent. Is this a bad idea? Do I need a seperate vent for my Black? I am trying to redo my plumbing as simple as possible and don't like the pipe running up the wall to the vent, Is it very difficult to run a pipe up through the wall in another location as the factory did in the kitchen?
Any thoughts on the matter would be great.
Thanks
Todd

Check out all the detailed photos on my web site listed just below. I have a gallery named for the Airstream Project.

[markdoane]

The only problem that comes to mind would be if you undersized the vent tubing to fit inside the wall, and then the when you open the 3" outlet valve it sucks your traps dry. I don't think thats a large risk if you design the vents properly.

I used 1 1/4" irrigation tubing (black polyethylene) for my vents, they fit inside the wall OK and are fairly flexible.

[TPdrums]

Suck the traps dry? I don't understand. Isn't that what you want to do when you empty the tanks?
The irigation tubing, it's flexible? How did you fish it up through your wall? Or did you remove the skin to get it up there?

[markdoane]

You don't want to suck the traps dry. Here I'm talking about the P-traps under the sinks and shower.

If the vent is undersized, when you open the drain valve on the graywater tank, it will create a vacuum in the tank. The vent(s) lets air into the tank so that a vacuum doesn't happen. If the water is draining faster than the vent can let air in, it will try to create another vent by sucking air in through the sinks, and suck the water out of the traps.

I wouldn't happen unless you had really full tanks and undersized, poorly laid out, or partially plugged vents.

The irrigation tubing is stiff enough that it wont collapse, but flexible enough that it will conform to the curve of the walls.

I installed mine while the interior panels were removed. You wouldn't be able to fish it through the walls, you need to cut holes in the longitudinals for the tube to pass through.

[TPdrums]

Oh, the p-traps. I didn't think of that. I will for the most part be hooked up to septic. I would think that my grey tank will be always open, and the black I will try to fill up a bit before emtying it once a week or so, to avoid solids drying in the tank. I'd really like to try and get that vent from the black tank through the wall and out the top in it's own vent though. The previous owner had the bathroom sink hooked up to the black tank too. Would it be acceptable to use a smaller vent pipe, if the toilet is the only thing running into the black tank?

[Janets Husband]

For added venting you can use an Auto-Vent. Take a look at these, they are not the only brand. Auto vents and automatic home sewer vents from FAMOUS PLUMBING SUPPLY

[TPdrums]

Interesting vents. They would probably work in conjunction with other vents and p-traps, but the black water tank doesn't have a p-trap on it as it is now. It vents from the tank straight up the wall and out the ceiling.
http://www.toddcreatedwood.com/asset..._IMG_32401.jpg
I'd like to do just that but want to try to get it into the wall and out a different place. As to suit my interior architecture better.
My other option is to join it to the grey water vent, but then there would still not be any trap between the black tank and the auto vent. The site says it vents at discharge with the vaccum created by the flush or drain. I guess what I am looking for is an answer to whether or not I can vent the black tank with the grey. Is 1 1/2" pipe enough to vent the grey tank with two sinks and a shower, along with the black tank that only has the toilet hooked up to it? Or should I try to get a second vent routed for the black?