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Old 02-07-2015, 08:13 PM   #1
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So I've reached the point in my renovation where I'm plumbing in the new grey tank. Definitely not a fan of putting holes in my roof, so I was wondering what the reasons were to not vent out the wall or even through the floor and belly pan. I'm assuming to eliminate odors near the windows? Has anyone done this?

Also, it seems like PVC is the preferred vent stack material. Is there a reason to not use vinyl hosing? Seems like it would fit the contour of the walls better. Thanks!
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Old 02-07-2015, 08:37 PM   #2
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Plumbing vents are designed to let air into your plumbing system. Yes, sewer gas generated in the tank exits the vent, but that it is a side effect.

The primary purpose of your vent stack is to allow air into the drain lines between the fixture's trap and the tank. This prevents the trap from emptying due to siphon effect.

So it must exit the trailer well above the flood level of the highest fixture (e.g. kitchen sink) or it will become an overboard drain when the sink drain line stops up.

Because the vent is on the tank side of the trap - there is nothing to stop stinky tank gases from coming out of it. The only way to keep that stink from wafting into a window or the nose of a passerby, is to exit it above the roofline.

Even if you manage to plumb the trailer without separate trap vents (using wet venting and tudor style air admittance valves) - you still will have the stink and potential overflow issues if you do not exit on roof.

I do not know RV plumbing code (it probably requires PVC or ABS to the top) but glued PVC would needed at least above the highest flood point in case there was a backup.
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Old 02-07-2015, 09:34 PM   #3
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Quote:
Originally Posted by calais66 View Post
...... it seems like PVC is the preferred vent stack material. Is there a reason to not use vinyl hosing? Seems like it would fit the contour of the walls better. Thanks!
As Wayward has pointed out roof vents are the approved method of tank venting.
I recall my '67 had ABS vent pipe that had been heated and bent to the contour of the shell before exiting the roof inside a molded cover.
I don't see any reason you couldn't use vinyl as long as you could secure, and seal, it properly to a solid roof vent.

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Old 02-07-2015, 09:54 PM   #4
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Also remember that when you drain the tank, a lot of air must come in quickly to replace the liquid which drains out. That requires a good sized vent line open to the atmosphere.

In addition, there is the "slosh" factor that means that as the water in the tank surges around as you travel, air may be forced in and out. If the vent is too low, the water in the tank will wind up sloshing out. So, no venting through the belly pan.

Bottom line is that there are very good reasons that the venting systems used in trailers is what it is, where it is.
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Old 02-08-2015, 05:47 AM   #5
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You should have a vent exiting the roof already from the sink, just tie your gray tank vent into that, no need for another separate pipe.
That's what I did in my 63 and it works great.
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Old 02-08-2015, 12:56 PM   #6
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Thanks everyone, I very much appreciate the feedback! So wall venting is definitely out. The idea for venting through the floor though would have a vertical vent pipe coming out of a T fitting where the waste line meets the tank, rising a few feet (above the level of the galley sink) and then take a 180* curve back down through the floor. This whole assembly could be hidden inside a bulkhead. I think that would eliminate the slosh factor but probably not the odor factor.

The only existing roof vents on this model ('51 FC) were the furnace (missing and patched on ours) and stove vent stacks. The galley sink originally drained straight down through the floor of the trailer and wasn't vented through the outer skin. I guess the attitude back in 1951 was the world was your grey tank
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Old 02-08-2015, 01:10 PM   #7
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Best place to vent is the roof. I had a scamp trailer and it was vented on the Side of the trailer and after a few days of using the facilitys boy what a smell sure glad our airstream is vented on the roof , also I have bent pvc pipe by heating sections and rotating them over my BBQ side burner it works great.

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Old 02-09-2015, 07:49 AM   #8
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Bending PVC

Here are some methods for putting smooth bends into PVC pipe.

Sand works fine but that Bendit tool looks cool
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Old 02-09-2015, 09:19 AM   #9
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I use flexible pvc all the time on swimming pools. Works great, no need to heat and bend. We use it primarily on spa plumbing where there are some tighter radius runs.

Call your local pool builder, if he is like me, he has plenty of scraps
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Old 02-09-2015, 02:07 PM   #10
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Awesome! Thanks for the suggestions. The sand method is one I'm definitely going to try (though it's hard to imagine a cooler name than the 'Pipe Viper'!).

Also, that's a great idea re. reaching out to pool and spa installers for flexible PVC. I'm a huge fan of that stuff, used it to plumb part of the grey tank's drain system, but buying at the Orange box store made it cost prohibitive. Thanks for the tip!
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Old 02-09-2015, 05:10 PM   #11
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Awesome! Thanks for the suggestions. The sand method is one I'm definitely going to try (though it's hard to imagine a cooler name than the 'Pipe Viper'!).

Also, that's a great idea re. reaching out to pool and spa installers for flexible PVC. I'm a huge fan of that stuff, used it to plumb part of the grey tank's drain system, but buying at the Orange box store made it cost prohibitive. Thanks for the tip!
Just don't skimp on size, again, remember when you dump the tank you need to replace the liquid with air at a very rapid rate (as well as code reasons)
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Old 02-10-2015, 12:27 AM   #12
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I'm a huge proponent of the Studor Vent or Air Admittance Valve. I installed these in my house and eliminated all venting through the roof. The valves open when suction is present i.e. water flowing down a drain or when draining a tank, then closes when suction is removed. No sewer gases are ever detectable and all drains and toilets empty without issue. I don't know what code is for an RV but Studor vents pass building code in both residential and commercial applications. I will be installing these vents in my airstream when I get my shell back on. Just make sure where ever you install the vent it has plenty of unrestricted air movement.
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Old 02-11-2015, 04:41 AM   #13
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a limitation using AAVs on an RV

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I'm a huge proponent of the Studor Vent or Air Admittance Valve. I installed these in my house and eliminated all venting through the roof.
Air admittance valves (AAV) are used in RVs too. AAVs can have a problem though. One (which I have had to deal with) is when a main plumbing drain line backs up. When that happens, raw sewage can find its way into the air admittance valve body that sits below the flood level of the fixture it vents. Particles of sewage can stick in the valve, causing sewer gas to leak into the house after the clog is cleared.

Although atmospheric roof vents are the best way to vent a house, AAVs can eliminate the roof vent, but an RV would still need a main tank vent even if all the drain traps were protected with AAVs.........

In a house on city sewer, air volume is brought in through the stack when a toilet is flushed or tub is drained. No air is displaced out of the roof vent because the sewage flows into an open sewer.

In an RV however, the water from the tub, shower, etc drain into a closed tank. Not only must air be admitted to protect he traps, but air must also be displaced from the tank. That has to to go out of the tank roof vent.
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Old 02-12-2015, 10:51 AM   #14
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In a house on city sewer, air volume is brought in through the stack when a toilet is flushed or tub is drained. No air is displaced out of the roof vent because the sewage flows into an open sewer.

In an RV however, the water from the tub, shower, etc drain into a closed tank. Not only must air be admitted to protect he traps, but air must also be displaced from the tank. That has to to go out of the tank roof vent.
Excellent analysis and advice. Venting may seem simple but it is not, especially when you have to consider each and every factor in the pressure as well as drain side. I had missed, in my mind, the draining into a closed tank aspect of the problem. Thanks.
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