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Old 04-14-2015, 12:41 AM   #1
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Black & Gray tank vent pipes - 3170DS

I have a 2013 3170DS. Went to change the vent cap on the roof for the black and gray water tank vent pipes. Could only find one vent/vent cap. Does anyone know if the gray water and black water vents are tied together on a 3170DS? I thought the norm was to have a separate vent pipe for each tank.
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Old 04-14-2015, 07:39 AM   #2
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Yes they are tied together. I thought it strange as well but it seems to work ok.
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Old 04-14-2015, 08:44 PM   #3
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Actually most houses will tie everything into one vent. I used to design for commercial construction. The fewer penetrations through the roof the better. There are all sorts of calculations about how to size the vent based on downstream and upstream drains, because some things can drain into a pipe that is venting lower fixtures. 3" being the minimum due to potential frost closure.
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Old 04-14-2015, 11:39 PM   #4
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Actually most houses will tie everything into one vent. The fewer penetrations through the roof the better...
I know on houses that they try to tie all of the plumbing venting together for the reason you mentioned (my house has only two plumbing vent pipes poking through the roof), but after researching about occasional smell from the holding tanks coming into the motorhome with the drivers window open while going down the highway... that it could be caused by the vent cap not siphoning the fumes away, I decided to get one of those swiveling, siphoning, venture type caps and try it. Everything I had read said I was going to find a separate vent pipe for the black tank and the gray tank on the roof and that I needed to buy 2 of these vent caps. Luckily I only bought one. I was surprised (and relieved) to find everything routed into one easily accessible vent pipe when I looked up on the roof. Made the job that much easier.
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Old 04-15-2015, 01:27 PM   #5
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I had the odor problem and installed a solar power fan cap on the riser. Have never had any odors after.
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Old 04-15-2015, 01:33 PM   #6
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I had the odor problem and installed a solar power fan cap on the riser. Have never had any odors after.
Not sure how .
An rv holding tank is a closed system.
The ONLY time that fan can work is the 1-2 seconds when your toilet flap is open to flush.
Otherwise the fan cannot draw air from the tank without introducing air from somewhere else.

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Old 04-15-2015, 03:56 PM   #7
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Not sure how .
An rv holding tank is a closed system.
The ONLY time that fan can work is the 1-2 seconds when your toilet flap is open to flush.
Otherwise the fan cannot draw air from the tank without introducing air from somewhere else.

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2015 camping "2 nights"
If there is a black tank cleanout that is where the air draw would be allowed. The backflow preventer is just a mechanical plunger that can be vacuumed open to the input port which typically is wide open. There would be no air draw from the grey tank. If there is no black tank cleanout then I think you are correct.
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Old 04-15-2015, 05:12 PM   #8
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I can only report I had such an odor problem and this is what I installed and it worked.
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