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Old 09-21-2021, 06:17 AM   #1
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Adding valves to fresh water overflow lines

We have been having issues where I fill up the fresh water tank at home then drive to the campground and are down to 1/2 to 2/3 of a tank of fresh water. Ran out of fresh water last weekend. I know why: it is siphoning out of the overflow lines (there are two overflow lines on the fresh tank). I can see it running out as I go down the road.

I am going to install a quarter turn shut off valve to each overflow line. My plan is to fill the tank until water comes out of the overflow lines, let them drain for 30 seconds or so and then shut off the valves.

My question is, should I open the valves upon arrival at the campsite? Temps can vary quite a lot this time of year and I don't want the tank to explode. For now, we are camping within an hour of home, so I think this should not cause any issues, but thought I would ask.
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Old 09-21-2021, 06:35 AM   #2
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Lots of us use both sites.....
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Old 09-21-2021, 06:37 AM   #3
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You should at least open one valve to prevent the tank from collapsing when the water is used.
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Old 09-21-2021, 06:45 AM   #4
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I've been considering adding a valve to mine, but what has kept me from doing it is the chance of rupturing the tank, or blowing off a fitting if I forget to open the valve when I fill it. I wouldn't worry about it if my tank filled via gravity, but it fills through city water inlet. My tank is in the main storage compartment so if something failed, it could flood 1/2 the RV.
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Old 09-21-2021, 06:52 AM   #5
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While a valve in the vent line IS an option (only with absolute diligence to reopening it) there is another option:


An anti-siphon valve (aka siphon break). Very common on boats. Really depends on whether you can lead the vent line up above the tank. This one is idiot-proof, needing no owner interaction once installed.
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Old 09-21-2021, 07:12 AM   #6
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Lots of us use both sites.....
I know, but this site has more than just Forest River owners and I get a wider variety of feedback.
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Old 09-21-2021, 07:13 AM   #7
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Originally Posted by wolfe10 View Post
While a valve in the vent line IS an option (only with absolute diligence to reopening it) there is another option:


An anti-siphon valve (aka siphon break). Very common on boats. Really depends on whether you can lead the vent line up above the tank. This one is idiot-proof, needing no owner interaction once installed.
This would be better. So the siphon break would need to be installed above the fresh water tank? That would be difficult.
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Old 09-21-2021, 07:22 AM   #8
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It's an annoying problem, for sure. But I would not put a valve on your overflow under any condition. It would only be a matter of time before you would forget it and have an issue.

If you forget about the valve being closed when pumping from it, you would create a collapsing force on the sidewalls, top and bottom. The pump would probably become unable to pull any more water before any damage would be done.

If you forget about it when you try to fill it, you would apply "city water" pressure to the inside of the tank and if any of the six sides of the tank are unconstrained, it could be fractured pretty easily. Example: Let's say your largest tank wall is 10" x 48". That's 480 square inches. Let's say your city water pressure is 60 psi. That adds up to a total of 28,000 pounds of force on the sidewall of the tank. I think that if you didn't notice it pretty quickly, failure would be certain.

I very strongly feel that it would not be a case of "if", but a case of "when".

Now, that said, it may very well be that if you did NOT fill the tank to the level of the overflow outlet, you may not start the siphon in the first place. You also may be able to shorten the down-tube on the overflow piping, some. That would limit the extent of the draining to the level of the bottom of the tube. But we found that when we didn't fill the tank to overflowing, we did not have the drainage problem that we had when we would fill it completely.

I think that our Dynasty may have had two such ports. I don't know, but it is possible that they each had a check valve with one allowing air in, and the other allowing water out. I suspect that may be the case. If there is just one without a check valve, the siphoning should stop fairly quickly as any "drainage" would begin to create a lower pressure inside the tank than outside of the tank as the water drained off and that would stop the siphon.
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Old 09-21-2021, 07:30 AM   #9
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It's an annoying problem, for sure. But I would not put a valve on your overflow under any condition. It would only be a matter of time before you would forget it and have an issue.

If you forget about the valve being closed when pumping from it, you would create a collapsing force on the sidewalls, top and bottom. The pump would probably become unable to pull any more water before any damage would be done.

If you forget about it when you try to fill it, you would apply "city water" pressure to the inside of the tank and if any of the six sides of the tank are unconstrained, it could be fractured pretty easily. Example: Let's say your largest tank wall is 10" x 48". That's 480 square inches. Let's say your city water pressure is 60 psi. That adds up to a total of 28,000 pounds of force on the sidewall of the tank. I think that if you didn't notice it pretty quickly, failure would be certain.

I very strongly feel that it would not be a case of "if", but a case of "when".

Now, that said, it may very well be that if you did NOT fill the tank to the level of the overflow outlet, you may not start the siphon in the first place. You also may be able to shorten the down-tube on the overflow piping, some. That would limit the extent of the draining to the level of the bottom of the tube. But we found that when we didn't fill the tank to overflowing, we did not have the drainage problem that we had when we would fill it completely.

I think that our Dynasty may have had two such ports. I don't know, but it is possible that they each had a check valve with one allowing air in, and the other allowing water out. I suspect that may be the case. If there is just one without a check valve, the siphoning should stop fairly quickly as any "drainage" would begin to create a lower pressure inside the tank than outside of the tank as the water drained off and that would stop the siphon.
The problem is it will stop siphoning while I am sitting at home and then start up again when I am rolling down the road. We were down to 1/2 tank the last trip. That ain't enough for momma.
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Old 09-21-2021, 07:33 AM   #10
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Quote:
Originally Posted by corn18 View Post
We have been having issues where I fill up the fresh water tank at home then drive to the campground and are down to 1/2 to 2/3 of a tank of fresh water. Ran out of fresh water last weekend. I know why: it is siphoning out of the overflow lines (there are two overflow lines on the fresh tank). I can see it running out as I go down the road.

I am going to install a quarter turn shut off valve to each overflow line. My plan is to fill the tank until water comes out of the overflow lines, let them drain for 30 seconds or so and then shut off the valves.

My question is, should I open the valves upon arrival at the campsite? Temps can vary quite a lot this time of year and I don't want the tank to explode. For now, we are camping within an hour of home, so I think this should not cause any issues, but thought I would ask.
I once has a Jayco 222SL travel trailer that did this. I was able to run a clear hose from each overflow line and I made sure there was a loop in each added line. (mounted as high as I could get it) Solved my issue
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Old 09-21-2021, 07:42 AM   #11
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Why do I have two overflow lines?
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Old 09-21-2021, 09:47 AM   #12
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Ok, valves are out. Going boondocking this weekend and really need as much water as possible. I looked at the drain lines and they hang down about a foot from the plastic stuff enclosing the under belly. I can shorten that up and then add an extension hose that runs up to the belly outside the I beam frame. That will put the drain height about 18 above where it is now. Ill see how that works. Certainly a lot better than hanging a foot below the bottom of the fresh water tank.
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Old 09-24-2021, 06:31 AM   #13
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Update

I put an elbow on the end of each overflow line and ran plastic hose from the elbow up to the floor of the trailer. Gained about 15 of height. Filled the fresh water tank until it overflowed. Stopped draining a lot quicker. Showed up at my campsite showing a full fresh water tank. So maybe the simple approach will work. Well see over time.
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Old 09-26-2021, 08:40 PM   #14
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2 overflows is part of the new anti-siphon solution.

Beware that too long a pipe full of water won't empty easily. I'm redoing my single extension to solve some of these problems.
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