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Old 09-21-2021, 07:17 AM   #1
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Adding shutoff 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, 07:24 AM   #2
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I would question adding the shut off valves. Are the over flow lines actually used as tank vent lines? Would you be better off installing a siphon break? Is this a valid complaint that could be handled under the warranty program?
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Old 09-21-2021, 08:15 AM   #3
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I would question adding the shut off valves. Are the over flow lines actually used as tank vent lines? Would you be better off installing a siphon break? Is this a valid complaint that could be handled under the warranty program?
They are used as vent lines. Another big issue is if I forget to open them to fill the tank. Ouch!
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Old 09-21-2021, 09:25 AM   #4
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I would not put valves on a vent line I solved this exact problem on my old class A by routing the vent lines (two fresh water tanks tied together) up higher then the tank and never had a problem again.
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Old 09-21-2021, 09:39 AM   #5
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I added a shut off valve....I also wait until the misses takes a shower to drain down the tank some before I open the valve. Otherwise the syphon effect would just happen at the campsite.
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Old 09-21-2021, 09:46 AM   #6
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Tanks are vented for very good reasons. Putting shut-off valves in vents is asking for trouble. Automatic syphon break check valves are the correct answer here. Or, as SeaDog and others pointed out - just raise the vent location.

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Old 09-21-2021, 09:54 AM   #7
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syphon valve, vent relocation, correct answer there is always more then one way to skin a cat. JMHO
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Old 09-21-2021, 10:47 AM   #8
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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. I’ll 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 09-21-2021, 11:35 AM   #9
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The only way it can siphon is for the atmospheric end of the vent line(s) to be below the water level in the FW tank, and the tank end of the vent line is submerged below the full water level. This needs to be corrected.
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Old 09-21-2021, 01:14 PM   #10
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Simple (and no risk) solution

With apologies to the draftsmen out there...
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Old 09-21-2021, 01:17 PM   #11
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With apologies to the draftsmen out there...
Attachment 263163
Love it!!! Not sure I'll be able to implement this solution before leaving Thurs. Rain, rain, rain. And I have to drop the silly underbelly stuff which I have never done.
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Old 09-21-2021, 01:25 PM   #12
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With apologies to the draftsmen out there...
Attachment 263163
only problem with your picture in my situation is the tank has its vent/overflow fitting into the side of the tank not the top.
Also unless you make a big loop (which I dont have much space above to the floor) you can still slosh water out the vent going down the road.
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Old 09-21-2021, 02:02 PM   #13
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Hmm...

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Originally Posted by asr524 View Post
only problem with your picture in my situation is the tank has its vent/overflow fitting into the side of the tank not the top.
Also unless you make a big loop (which I dont have much space above to the floor) you can still slosh water out the vent going down the road.
Hmm. Can you run flexible tubing over to a wheel well and arch it above the tank level there? Anyplace else where you can arch it up? Maybe up through the floor and back down if a storage area is nearby?
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Old 09-21-2021, 02:10 PM   #14
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Originally Posted by D W View Post
The only way it can siphon is for the atmospheric end of the vent line(s) to be below the water level in the FW tank, and the tank end of the vent line is submerged below the full water level. This needs to be corrected.
Is it possible for the tank sloshing on the road or the wind on the end of the tube creating some kind of suction effect to suck the water out while driving. Just curious, I put a shutoff valve on my overflow line but only gravity fill my tank.
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Old 09-21-2021, 02:16 PM   #15
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Hmm. Can you run flexible tubing over to a wheel well and arch it above the tank level there? Anyplace else where you can arch it up? Maybe up through the floor and back down if a storage area is nearby?
I put in a shut of valve....post #5
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Old 09-21-2021, 02:51 PM   #16
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Originally Posted by asr524 View Post
only problem with your picture in my situation is the tank has its vent/overflow fitting into the side of the tank not the top.
Also unless you make a big loop (which I dont have much space above to the floor) you can still slosh water out the vent going down the road.
Understood; but any sloshing would be intermittent and minimal compared with syphoning.
My vent connections are also in the side just below the top - sketch was schematic.

You don't need much of a loop (or some 90s, if overhead is limited) to get the level of the discharge above tank level; then dump it into an open PVC running down to below the floor - end of involvement.

If you valve the vent and ever forget it, your tank could collapse, as others have pointed out.
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Old 09-21-2021, 02:59 PM   #17
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Is your fresh tank gravity fill or pressure fill?
If gravity fill, add the valve and stop worrying. Gravity fill cap is another vent. Most folks do this for the same reasons.
If it's a pressure fill, then you just have to be smarter than what you are playing with and remember when to ensure the valve is open.
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Old 09-21-2021, 03:19 PM   #18
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Originally Posted by NMWildcat View Post
Is your fresh tank gravity fill or pressure fill?
If gravity fill, add the valve and stop worrying. Gravity fill cap is another vent. Most folks do this for the same reasons.
If it's a pressure fill, then you just have to be smarter than what you are playing with and remember when to ensure the valve is open.
X2

If adding shut off valves is playing with fire, you just need to remember who the fire starter is. The valves were added for a reason. Why when you get to the campsite would you suddenly have a loss of memory as to why you added them?

Now... if you add them, close them and sell your R/V to someone else, then that could lead to trouble.
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Old 09-22-2021, 05:49 AM   #19
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Bad Idea me thinks

If you have a shower miser on your trailer you could blow up your fresh water tank if you forget to open the valve.
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Old 09-22-2021, 10:12 AM   #20
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Hmm...

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If you have a shower miser on your trailer you could blow up your fresh water tank if you forget to open the valve.
Hmm...I have to think about this for a minute.

If you are operating from your fresh water tank, you would be putting water back into the tank at the same rate you are drawing from it, right? No issue there.

If you are operating from city water, and running the water miser by mistake, that's a similar situation to filling with the valve closed and could inflate the tank.

Not sure why someone would run the water miser with city water, but someone is bound to do it.
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