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Old 11-23-2017, 09:59 PM   #21
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Originally Posted by scottz View Post
I also don't fully understand the purpose or need for this anti siphon device unless just to satisfy a code. I don't see how liquid in the un-pressurized black tank could ever make it's way into the pressurized water hose.

And why an anti-siphon device with an air vent inside the trailer instead of a simple check valve?
You need to have an anti-siphon to break the static water path for the bacteria. The black water may not travel back through but static, non moving water provides a virtual freeway for bacteria.


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Old 11-23-2017, 10:32 PM   #22
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The anti siphon valve is also an air gap, so when you finish flushing the tank, and disconnect the hose to the flush inlet, the air gap will open and drain the water on the tank side to the tank, and the water on the in-coming side back out the the inlet fitting. The air gap/anti siphon valve has to be installed at a level higher than the top of the toilet bowl, so even if the black tank filled so high that it backed up into the toilet, it still could not pressurize the flush line to back sewage to the flush inlet. Because the flush line is totally isolated from the fresh water system, and assuming the hose is never left connected to the flush inlet, it is virtually impossible to cross contaminate to the fresh water side. When the valve is eliminated and someone uses a check valve at the flush inlet, the water never drains from the flush line, so if the tank did overfill, it may be possible for the flush line to siphon sewage, but it still can't because there is a check valve on the inlet. The only way that a flush line full of water can cause a problem is in freezing temps, the flush line would also need to be winterized.
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Old 11-24-2017, 06:35 AM   #23
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Ya know I didn't think about freezing when I hooked up my hard pipe black water flush line! At least I did run anti freeze thru the line! Makes me rethink if I want to go back and use a hose that can be disconnected as opposed to pvc pipe.......
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Old 11-24-2017, 09:56 AM   #24
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Scottz - The Tech explained to me that if the black tank backed up to the point that it reached the anti-siphon valve then it would release out into the sink cabinet after the diaphram inside the anti-siphon valve closed off the city water connection. That was 8 years ago, so I don't know if they're made that way today or not.

When the black tank fills up, you're not going to know it unless you flush it and see that it has filled the pipe connected to your stool. But when you connect a city water line that has 60 lbs or more of water pressure to your backflush connected to your black tank which may or may not be full, your tank can fill up very fast. There is no safety device that shuts off the backflush if your drain valve is closed. The plumbing system does have a vent thru a pipe that extends out the roof of the camper. If something happened that caused you to forget to open your drain valve while the backflush was running, I suppose you would eventually see sewage water spouting out the roof. Why the anti-siphon valve wouldn't just be a check valve rather than a diaphram that released it into the camper is beyond me. I'm just repeating what the Tech told me.

The anti-siphon valve really is necessary. Don't ever bypass it as some have suggested. City water faucet connections can be various designs. They can be just a valve connected to a water line or a freezeless faucet like you see on the farm where you lift the handle for on and push it down for off. The freezeless is screwed onto a pipe that goes into the ground and connects to the water line below the frost line 36-40 inches below ground. When you shut the faucet off it allows water in the pipe from the faucet down to the connection below the frost line to drain out into the ground. This prevents the faucet from freezing. If you left your backflush hose connected after shutting the water off you could be siphoning waste water from the black tank back towards the faucet. That single source of fresh water feeds both your fresh water and the backflush. Maybe you don't color code your water hoses - black for backflush and white for fresh like I do. Maybe one camping event you reverse which hose is used for which purpose and maybe just a little contamination occurred. Your stomach will tell you when this happens.

And no, I have never had my black tank fill to the point that it drained into my camper. But you all know how it is when you've stayed 3 months and made a lot of friends and are trying to leave and drain your tanks one last time with all of those friends wanting to wish you farewell and oh, by the way, you turned on the backflush and forgot to pull the drain valve. Or in the rare occasion your tank plugs and you don't have a clear plastic connection to see what's comming out and it fills.
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Old 11-24-2017, 10:07 AM   #25
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Wow! Great info here. I'm changing how I flush my black tank. Learned a lot.

Thanks whutfles
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Old 11-24-2017, 10:24 AM   #26
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It's is more a pro cushion so that siphoning does not happen more will moving down the
road then will siting still. I took my valve out because it started to leak because they are
so cheep the one they put in at the factory that is you can buy a good brass one I allso
cut the lines off under the floor and the valve lives down under in the belly of the 5er
wear it should be.

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Old 11-24-2017, 09:38 PM   #27
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I think I got it now; thanks.

Looks like this would be a good choice to replace the plastic valve.

It would require some adapters.
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Old 11-25-2017, 01:33 PM   #28
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After reading some of the comments here, I am even more committed to only drinking bottled water when camping. People who don't understand backflow and ignorantly remove or bypass the anti-siphon valve are a health hazard. IMHO
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Old 11-25-2017, 02:18 PM   #29
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And another turnoff is when I pull into a site and the fresh water faucet is near the ground and within a foot or two of the sewer connection. I've seen peoples sewer hoses break and leak out and it's a mess.
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Old 11-25-2017, 09:00 PM   #30
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Originally Posted by Bad Moon View Post
After reading some of the comments here, I am even more committed to only drinking bottled water when camping. People who don't understand backflow and ignorantly remove or bypass the anti-siphon valve are a health hazard. IMHO
I know there are a lot of people who have elaborate filter systems on there rv's for the water but i never drink the water and use nothing but bottled water for drinking as i do at home. I put a inline filter on the hose which i think suffices for anything else i might use the water for. I use a separate hose for black tank flush and never leave the black tank hose on the rv or faucet when finished flushing.So for so good but you can't be to careful.
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