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Old 11-29-2009, 12:39 PM   #1
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Holding tank vents?

I don't know about other members, but IMHO Airstream went a little overboard in their the number of holding tank vents in the 70s. Maybe it was the code.

My '75 Sovereign had two vents for the gray water and tank. The first one was a drain vent for the J-traps. It was in a false wall between the kitchen and shower. I replaced that vent with one of those one-way vent valves, which is now hiding under the counter.

The other one is here. This shows the remnants of the double closet (another waaay too big of an item). You can see the vent pipe running up what was the corner of the closet. I'm pretty sure this is for the gray tank, since there is a black vent running up the corner of the bathroom.

Click image for larger version

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My question is in two parts. First, wouldn't the vent in the drains in the kitchen be sufficient to supply air for emptying the tank? Maybe the flow would be reduced somewhat, but that's a small price to pay for getting rid of the vent pipe and getting 3" of counter top in return.

Second, since the existing one-way vent valve only allows air to flow into the pipes, is there a problem with gray water producing gas that needs to be allowed to escape? This would only be a problem when you have the dump valve closed, as in boondocking. I mean, gas in significant enough quantities that it might bubble back past the water traps in the sinks or shower?

I'm itching to get rid of this vent and install single seat and additonal small work table there, with the user facing aft.

Zep
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Old 11-29-2009, 01:10 PM   #2
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Originally Posted by Zeppelinium View Post
I don't know about other members, but IMHO Airstream went a little overboard in their the number of holding tank vents in the 70s. Maybe it was the code.

My '75 Sovereign had two vents for the gray water and tank. The first one was a drain vent for the J-traps. It was in a false wall between the kitchen and shower. I replaced that vent with one of those one-way vent valves, which is now hiding under the counter.

The other one is here. This shows the remnants of the double closet (another waaay too big of an item). You can see the vent pipe running up what was the corner of the closet. I'm pretty sure this is for the gray tank, since there is a black vent running up the corner of the bathroom.

Attachment 91632

My question is in two parts. First, wouldn't the vent in the drains in the kitchen be sufficient to supply air for emptying the tank? Maybe the flow would be reduced somewhat, but that's a small price to pay for getting rid of the vent pipe and getting 3" of counter top in return.

Second, since the existing one-way vent valve only allows air to flow into the pipes, is there a problem with gray water producing gas that needs to be allowed to escape? This would only be a problem when you have the dump valve closed, as in boondocking. I mean, gas in significant enough quantities that it might bubble back past the water traps in the sinks or shower?

I'm itching to get rid of this vent and install single seat and additonal small work table there, with the user facing aft.

Zep
Zep.

There are a number of reasons for additional vents.

They keep airborne contaminants separated.

One tank is full, the other is not.

The distance from the vent, to the source.

How the trailer may be tilted.

I also think that the RVIA has a big hand in it, as well, to comply with basic health standards.

I assure you, that Airstream would not install an tank vent, unless it was absolutely necessary.

You the owner, however, can alter anything, anyway, you wish. Could that become an issue down the road? And to who? Who knows.

Andy
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Old 11-29-2009, 03:52 PM   #3
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Definitely need the stack. If nothing else double wall it. It's only 1 1/2" not including wall material.

..........just a thought from a builder....


Al
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Old 11-29-2009, 07:24 PM   #4
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Double walling may happen, but it doesn't get rid of the stack, which is in the way. Curving the stack against the wall may help, but eliminating it is the objective.

Here's the original Airstream gray tank system. The drains are vented and the tank is vented separately. However, the vent on the drains can also vent the tank.

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If the tank had a cross section that looked like this, it would require a second vent in order to utilize the full volume. This is only a problem for filling the tank, not draining it.

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My proposed modification would look like this, using the new concealed vent valve to allow air into the system, but not let it out.

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I now see the problem. Darn. If the original kitchen vent stack remained open to the atmosphere, the second stack would probably be helpful but not necessary. In a house, where there is the potential for a full bath tub to drain, or toilet to flush, quickly into the drain pipes, this is definitely a different matter. A test may be in order when the weather warms up!

OK. I agree I need an open stack. But two?....

Zep
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Old 11-29-2009, 07:54 PM   #5
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Zep

The vent valve will work, just as in conventional construction. I'm sure you know, but some may not, that the vent after the trap is to pull air in to the system so that a vacuum produced by the flushing water is equaled. With no vent stack or AAV the water after the trap would sit and not flow into the tank. (back pressure) The tank vent is a must! Think of it as the exhaust pipe of the entire system. The common mistake made is that people try to connect two vents together in the wrong way. The correct way is to think of a "T" upside down that both vents connect to and then vent to the outside. The reason that older Airstreams (and homes) don't have AAV (air admittance valves) is they were not around. Note that the AAV needs to be placed 12" above the the level of the trap, So you will still have a pipe that would extend about 30" above the floor.

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Old 11-29-2009, 09:49 PM   #6
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Zep, why not take from the design idea used in our 63 safari for the kitchen sink vent. It was a 1" piece of ABS (could be redone in pex or similiar material) and run it in the outside wall between the skins. You'd have to drill a few rivets to fish it through, but could use a barbed connector to 90 out the roof, and 90 out at the floor and run in the toe-kick area to meet up with your original floor location.

Just a thought.
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Old 11-29-2009, 09:51 PM   #7
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I have the AAV at my galley, along with the rear stack. I understand what you are saying, but you need one that vents outside (above the roof). If you dont have a vent to the atosphere, those gases are going to find a way in to the interior, and can be dangerous.

If I chose one to eliminate, I'd lose the stack over the piping and install an AAV but leave the tank vent...

Perhaps you can move it to the opposite side of the wall? or will that help any.....

Al
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Old 11-30-2009, 04:48 AM   #8
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Zep,

I'm thinking that is your black tank vent. The one in the curb side corner of the bath (rear bath) is the second grey vent.

In my 75, the kitchen vent was ran up between the outer and inner skins through a hole in the inner skin. If you look at my blog pics, you can see a couple of the vent pipes still hanging from the roof.

I'm assuming that you have a rear bath as well. My grey tank was rear curb side and black tank was rear street side.

I'd go out and take another pic, but my A/S is in Mississippi and I'm in Guam
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Old 11-30-2009, 08:40 AM   #9
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Quote:
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I'm thinking that is your black tank vent. The one in the curb side corner of the bath (rear bath) is the second grey vent.
No, it's a center bath. I'm pretty sure that the black vent is in the corner of the bath--when you open the toilet valve (you may recall I also had to take the toilet off and fix a crack in the tank some time ago) you can see the black tank extends under where the vent pipe is, in the bathroom.

Quote:
In my 75, the kitchen vent was ran up between the outer and inner skins through a hole in the inner skin. If you look at my blog pics, you can see a couple of the vent pipes still hanging from the roof.
Exactly, mine too. I may just remove the valve and run a new vent up the wall, curving it to fit. It'll be right in the corner of the kitchen and the shower partition. I learned how to heat PVC and curve it from a hot tub installer in Vegas about 25 years ago. I've done this for the rear vent in the Overlander and it looks pretty good without having to enclose it in a false wall.

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I'd go out and take another pic, but my A/S is in Mississippi and I'm in Guam
Thanks, and LOL here. You sound like AEROWOOD, who always seems to be a world away when I need him.

Zep
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Old 11-30-2009, 09:57 AM   #10
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Thanks, and LOL here. You sound like AEROWOOD, who always seems to be a world away when I need him. Zep
I'm home, really I'm home, for a little while at least. I actually worked on the water tank platform Sat.
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Old 11-30-2009, 11:49 AM   #11
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As water goes in ,air must go out,and an AAV will not let air go out,so yes you need the other vent. Dave
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Old 11-30-2009, 12:05 PM   #12
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A person could re-route and/or reduce the size.
It only takes a "sip of air" to drain and a "way of escape"
to vent. They vent automotive batteries through
a straw-sized tube.

Just a couple of thoughts
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Old 11-30-2009, 12:35 PM   #13
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As water goes in ,air must go out,and an AAV will not let air go out,so yes you need the other vent. Dave
Yep, that's the conclusion I came to myself, but I had to draw it in order to see my lack of comprehension.

I think I'm going to replace the AAV with a somewhat smaller vent pipe than the original and remove the other vent completely. I have to say that it baffles me that the code doesn't seem to make any allowance for the difference in flow rates, etc., in an RV.

Zep
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Old 11-30-2009, 01:03 PM   #14
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I am going to use polypipe, you know that black stuff that we use here in Colorado for our sprinkler systems. It comes in several sizes, I think I got 1 1/2" and is semi flexible. I am running it in the exterior walls.

Kip
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