View Poll Results: TO VENT OR NOT TO VENT?
YES - vent the gray tank through the roof 18 94.74%
NO - just keep the tank clean and it will be fine 1 5.26%
Voters: 19. You may not vote on this poll

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Old 10-01-2015, 10:08 AM   #1
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Question gray water tank - does it really need a vent?

I am installing a 9 gallon gray water tank in a 1950 Flying Cloud. The tank will sit under the galley sink and will be used solely for water from the sink. I do not expect to use the sink for much heavy dishwashing. It will just be used for light rinsing here and there.

I am wondering how crucial it is that my new gray water tank have a vent plumbed through the roof or side of the trailer?

I think it is likely that I will keep my tank pretty clean and I will not be using it much. If the vent is mandatory then I'll bite the bullet and drill a hole in my virgin exterior. But if I could get away without a vent I'd love to keep my exterior intact.

All suggestions will be appreciated!
Thanks!

Ryan
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Old 10-01-2015, 10:28 AM   #2
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If you don't vent it somehow, you are going to get occasional bursts of sewer gas into your trailer. Depending on how you use the sink (and your reported use is as benign as possible) you might be able to get away without one. But to be safe, I'd find a way to vent the grey tank to the outside. Have you considered looping the vent stack back under the trailer so it's less obvious? This will not be as effective as a straight up vent, but it's probably better than letting the gas escape into the trailer.
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Old 10-01-2015, 11:34 AM   #3
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The main reason for venting has nothing to do with tank cleanliness or odor. It has everything to do with the fact that the tank is not pressurized.

You start off with a gray tank full of air. As water runs in, air has to go out somewhere. When the tank is full of water and you go to dump it, as the water runs out, air has to get in from somewhere. The vent takes care of letting air into and out of the tank.

It's as simple as that.

Now, the reason for venting to the outside, though the roof, is because the tank IS a source of odors, so you want it vented to outside rather than to inside the trailer.
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Old 10-01-2015, 09:26 PM   #4
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The Airforums are a wonderful resource! Thank you Bob and Protagonist for helping me understand my situation with the gray tank. I like the idea of a vent that loops back under floor. I'll look into that. If not, I guess a small vent in the roof is no big deal.
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Old 10-02-2015, 11:21 AM   #5
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If you are close to another "vent" going thru the roof, tie together up above the level of the sink.

Another less desirable option would be to vent from top of tank thru the floor... But that can cause more critter issues.
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Old 10-02-2015, 01:02 PM   #6
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You can install a cheater vent under the sink....most Airstreams have one under the bathroom sink. It is a spring valve or lightly weighted check valve. Get it at your plumbing supplier; Saves cutting holes through the aluminum skin. It only vents when there is water going into the tank. I have never had a problem with odors from this vent It is needed to let the water drain through the trap in the sink. Without it your sink will not drain or it will take a long time to dribble around the air lock under the trap.

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Old 10-02-2015, 01:33 PM   #7
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With under sink you will need to monitor tank level closely.
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Old 10-02-2015, 01:49 PM   #8
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Maybe a giant 2" PVC vent is not preferred, but no vent is an issue too. Split the difference? Maybe run a PEX sized vent tube up through the walls to the roof refrigerator vent?
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Old 10-02-2015, 02:31 PM   #9
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I would think the check valve under sink or tie into your bathroom vent if they're on the same side.
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Old 10-02-2015, 03:16 PM   #10
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The under sink vents also should be 6.00" minimum above the trap.
Or at least that is what is recommended.
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Old 10-02-2015, 04:01 PM   #11
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It will also empty more slowly without a vent as air will have to overcome the P-traps.
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Old 10-02-2015, 04:55 PM   #12
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The air admittance valves that are installed under the sink are only there to allow air into the drain above the p-trap so as water flows from the trap to the tank, air is allowed in to help with water flow and keep the trap from having all the water sucked out of it while the sink is draining. It will do nothing to vent the air out of the gray tank, or allow air to enter the gray tank while draining it without sucking the water out of the trap first. The tank needs to be vented. No, it doesn't need a 2" vent pipe, a flexible 1" pipe will work just fine. We tied our black and gray tank vents together, so we only have one vent pipe going out the roof.

Chris
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Old 10-03-2015, 09:23 AM   #13
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Did you consider using a sink insert to achieve a similar outcome (with a lot less work)? Perhaps you did, but I didn't see it mentioned here so I thought I would offer that idea as an alternative to a sink-only gray tank.

I use the 2-gallon nylon camping sink shown in the attached photo to capture sink water so it doesn't fill up my existing gray tank, which is very small. The photo to the right shows a polyethylene basin used by blogger Brent Rose to achieve the same thing in his Class B. Just lift by the handles and deposit the water outside.
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Old 10-03-2015, 04:32 PM   #14
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Re my post number 6...My Bad...Minno's right. It is an air admittance valve not a vent valve so it will not vent the tank. I like the idea of routing a small line up under the sink and then back down through the floor. That way there is little chance of water draining through the vent

Re post number 13..That's what we do to take the load off the Grey Water storage, We keep the bits and pieces of water in a tub in the sink, add hot water at supper to wash the dishes and then ( if possible...not always so...) water the 'flowers' with it

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