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Old 07-30-2016, 12:23 PM   #1
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Low Tech But Works

Cured the siphoning issue after filling the water tank by installing 90 degree elbow (Shark Bite) on the forward vent line, added a length of Pex to the LPG bay door and a second PVC elbow. When filling the water tank, I can now easily see when the tank begins to overflow. After turning the water off, I remove the PVC elbow and cover the end of the vent with a small piece of plastic wrap and push the elbow over the wrap to hold it in place. Siphoning problem solved.

My siphoning problem was extreme. From where the coach is parked to the highway I have to come down a steep hill. Parts of it are 1st. gear steep! During the 10-minute trek down the hill I estimate I lose 30 gallons. After reading of an owner that placed a valve on the vent line and then forgot to open it which resulted in a damaged tank, I could easily see myself making the same mistake. So idiot proof and safe solution was the goal. With my low tech solution, if I forget to remove the plastic from the end of the vent tube during a fill, the pressure would cause the thin plastic to give way long before damaging the tank. That is the theory anyway. Never intend to put it to the test.
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Old 07-30-2016, 03:06 PM   #2
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Good idea! Thanks for sharing.
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Old 07-30-2016, 03:41 PM   #3
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Quote:
Originally Posted by AKOne View Post
Cured the siphoning issue after filling the water tank by installing 90 degree elbow (Shark Bite) on the forward vent line, added a length of Pex to the LPG bay door and a second PVC elbow. When filling the water tank, I can now easily see when the tank begins to overflow. After turning the water off, I remove the PVC elbow and cover the end of the vent with a small piece of plastic wrap and push the elbow over the wrap to hold it in place. Siphoning problem solved.

With my low tech solution, if I forget to remove the plastic from the end of the vent tube during a fill, the pressure would cause the thin plastic to give way long before damaging the tank. That is the theory anyway. Never intend to put it to the test.
AKOne
That's not something I would recommend.

On my coach that "overflow vent" is also the ONLY vent to allow make-up air into the water tank when water is being drawn out of the tank by the water pump.
When I plugged that vent my water pump was strong enough to collapse the water tank enough to cause it to crack/leak.

It's possible that a "plastic wrap plug" would have failed before my tank collapsed... but I won't be taking that chance.

Mel
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Old 07-30-2016, 11:51 PM   #4
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AKOne,

Many of us have done something similar to the front overflow line but use a ball valve on the end of the new line. I even replaced the small screen fitting after the valve. I open the LPG compartment door to open & close the valve. Otherwise, the procedure is pretty much identical.

There is a rear overflow drain that actually attaches to the top of the tank as opposed to the side which keeps it from siphoning like the front overflow. NEVER modify that rear drain to close it off or you forget to open it and cause the tank to crack & leak. I have forgotten to open the front overflow valve before realizing that water was dripping out under the middle of the coach. No damage done.
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Old 07-31-2016, 08:28 AM   #5
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Hi AKOne

I am probably the one you are referring to that has the dubious distinction of rupturing my water tank because I had put valves on both vents and forgot to open them when filling! Now ,after repairing the damaged seam with Flex Set (thanks EM) I ALWAYS leave the rear vent valve open. When filling I open the front valve (I have reminders everywhere!) and start filling. When the front overflow start dripping I turn off the water. This way I get a little more in between the front and rear overflows. I try not to let the rear overflow as it will for some reason siphon about as much as the front overflow will. If I do forget to open the front valve the rear valve being open ALWAYS will allow the water to overflow through the rear vent.

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Old 07-31-2016, 08:48 AM   #6
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why not put a check valve on the vent line

not sure of the size you have for a vent, but an example of check valve is like
ABS-3-8-plastic-diaphragm-check-valve http://www.aliexpress.com/item/ABS-3...321905663.html
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Old 07-31-2016, 08:57 AM   #7
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Hi Kerryvan

I had installed a check valve that would open if the pressure got to a certain point on the front vent BUT I either installed it incorrectly or it failed to open when the tank was full. This is why I went with the more "interactive" method of 2 manual valves!

Geoff
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Old 07-31-2016, 09:40 AM   #8
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I also tried the check valve approach but with mixed results. By design, the std check valve is uni-directional--problem is here is you need overflow to escape but not allow water to syphon. You also need air to enter vent as pump empties the tank. Manual valve probably works the best but now you have to remember to turn it on/off. The other issue for me with any permanent valve or pipe extension is the prospect of stale water getting trapped in the vent line between uses--agree, not much but still and issue.
PS--know some folks only use on-board water for showers, dish washing, etc., and use bottled water for drinking, coffee, cooking, etc....
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Old 08-01-2016, 12:37 AM   #9
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Geoff, I certainly saw myself forgetting to open a manual valve. Your experience and willingness to post what happened is very much appreciated. It caused me to realize that whatever solution to stop the siphoning, I had to come up with one that would not cause damage if I missed a step. My plastic wrap scheme seems to be working fine.

Thought about Old Scout's concern regarding water sitting in piping and going stale, I decided to remove the plastic once the tank level drops to a point that siphoning is not an issue and let the standing water in the vent run out. Of course each time the tank is filled, fresh water is flushed through the vent line. Since we do a lot of dry camping, I am happy to not be losing so much water. Now I am trying to figure out how to increase my fresh water carrying capacity.
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Old 08-01-2016, 12:41 AM   #10
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Bob, thanks for the heads-up on the rear vent. Intend to leave it unmodified.
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Old 08-01-2016, 08:57 AM   #11
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6 years ago I devised an anti-siphon device, (using a check valve and a ball valve), which automatically prevented fresh water tank siphoning.

In worked perfectly fine for 4 years, (28k miles)... (until the winter of 2006 when I neglected to drain it when I winterized my coach for Wisconsin winter storage).

IMO if there was a simple/safe/foolproof way to preventing that siphoning RV builders would have figured it out by now...(and would be offering it as another expensive/overpriced option).

Mel
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Old 08-01-2016, 09:12 AM   #12
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I thought I read on here that somebody had run the over flow line all the way back to the wet bay, brought it out through the panel and added a valve there. Sounds like a good way to keep from forgetting about it. Siphoning is another issue I'll address when I get done with the big dollar drivability problems. It's nice to have a full tank for dry camping, even if it's only for a week end.

Terry
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Old 08-03-2016, 11:47 AM   #13
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On our 99 we did what Terry stated in the previous post. I lowered the tank to reach the front vent, and attached a new drain line and extended it to the rear service bay. I put a valve on the service bay panel, and dropped the drain through the floor. It is convenient and near the water fill. The back of the tank has a second overflow drain that loops up by the shower stall, before draining out through the floor, no valve on this line. Plenty of rise to avoid a siphon effect.
Open the valve, fill your tank, when water comes out this front overflow, close the valve, shut off the water, and you're good to go.
Ed
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