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Old 07-12-2003, 12:06 PM   #1
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grey water plumbing vent

I'm installing black and grey water tanks (had none originally), and am trying to figure out how to route plumbing vents. There's a good place to run a vent pipe for the black tank, but not for the grey. I had hoped to run it up through a closet, but there isn't one close enough. My question: can I connect a vent pipe to the grey water drain line (sort of like house plumbing), and run it up to meet the vent pipe for the black tank, or do I need a separate vent pipe out of the grey tank? It's a 12 gal. tank.
Doug
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Old 07-12-2003, 08:04 PM   #2
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You probably shouldn't connect the two tanks with a single vent because the drain lines are likely 3", but the vent is probably only 1-1/4".

Depending on how you built the drain lines, there may not be enough relief air through the single roof vent, and the draining will not "swirl" in the tank as it drains. This will eventually cause grime to build up.

You may want to think about venting the gray tank separately with an air admittance valve (AAV). Install the AAV at a height 4" higher than the highest drain, your sink, so if you fill up the 12 gallons in the gray tank, and your drain lines start filling, it will flow up into your sink before the AAV. You have to install the AAV in an area that can get air, and be accessible for maintenance if necessary, but it is a lot better than cutting a hole in the roof when you are trying to find ways to vent the drains.

Plumbing vent codes are available online, look some up if you aren't familiar with their specific requirements such as elbow types, horizontal venting, riser types.

Here is info on AAV's: www.rectorseal.com or www.studor.net

enjoy,

christopher
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Old 07-13-2003, 07:21 PM   #3
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Christopher,

Thanks for the info. about the air admittance valve. It looks like just what I need for the grey water tank.

Now I'm wondering if an AAV could be used to vent the black tank too, or is this a different venting issue? It would be great not to have to cut any holes in our old trailer.

Doug
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Old 07-13-2003, 09:11 PM   #4
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i assume you have at least one vent through the roof, for your sink, right?

I suggest tying the at least one of the tanks into that, with perhaps the other on an AAV, to give you added air relief for smooth draining. You simply won't have enough relief with just a single 1-1/4" vent for a big ole 3" drain line. AAV's come in various sizes, so just size it right for the job, whether it is for the black or grey.

The reason you want the AAV to be ventilated and accessible, is that the seals can go bad after a few years. Just to ease your conscience, you'll know an AAV is going bad when smells come into the interior. If that happens, simply cut off the old AAV and replace it.

Another thought for running the tank vents, is with flexible PVC pipe. It makes it easier to route along the curves of the airstream... just make sure you still have positive drainage for the vent pipe. Flexible PVC can also be glued, cut, etc. as normal Schedule 40 PVC pipe, and it is fine for DWV systems. Here's a link: http://www.plumbingworld.com/spaflex.html

christopher
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Old 07-13-2003, 09:37 PM   #5
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Question?

Why shouldn't you join the two vent lines. You only dump one tank at a time anyway and that tank already is vented through a 1 1/4" line. Venting the second tank through the same line should make no difference unless you pull both handles at the same time (and we all know better, don't we?).
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Old 07-13-2003, 09:57 PM   #6
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Both vent lines would be alright, if the combined stack went through the roof and was otherwise designed not to siphon out the water seal of the sink P-trap.

It may be alright with just AAV's, too. There are projects that use only AAV's, and no vents through the roof, but my conservative nature suggests that a combination should be used, since AAV's are subject to failure. They are easy to fix, though.

christopher
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Old 07-14-2003, 03:03 PM   #7
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This is really helpful; I'm getting a better sense of how the plumbing vent thing works.

Our trailer originally had no plumbing vent, no traps, and no holding tank. All the waste water drained into a park hook-up (or onto the ground). An old brochure suggested digging a hole under the toilet outlet and lining it with a paper bag for sewage!

So we're having to update some of the mechanical systems. I'd like to keep the ventless exterior of the old trailer, but it sounds like I should have at least one vent, then maybe combine the vent pipes or use an AAV for the grey tank. At this point, I think the AAV would be simpler and it could be easily accessible under the kitchen cabinet. I'll see how it goes once I get started on the installation.

Thanks for the info.

Doug
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