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Old 06-18-2022, 07:03 AM   #1
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Losing freshwater from the tank overflow valve

On a number of occasions I have filled our freshwater tank until the light is somewhere between flickering and steady on the full mark of the control panel. But upon arriving at the next location we always seem to between 1/3 and 2/3 full - with no apparent leaks in the camper. Water pump is off and faucets are off. Are we really losing that much water from the overflow valve due to sloshing around as we drive? I guess I had envisioned baffles in the tank, but more likely the cheaper nature of our rigs means no baffles are present.

If I am losing that much on the road, would it be okay for me to glue up a valve at the bottom of the pvc drain tube? Seems logical I’d have it open literally all the time “except” when driving on the road. (Need that checklist reminder to be in all caps!)

What does everyone think? Is the overflow drain my likely culprit? Is a valve the way to go?
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Old 06-18-2022, 07:27 AM   #2
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Does this unit have a gravity fill? If it does and is similar to our unit, I don't see how water could overflow from sloshing. Our trailer just has a vent tube near the fill tube opening. The only thing under the trailer is the fresh tank drain valve. We had some water at first dripping around there when the tank was full, but it turned out it was a leak where the fill tube connected to the tank. (The dealer has taken care of this). When I first filled the tank to sanitize, I thought the water dripping out near the drain must have been an overflow. Turns out it was just leaking out of the fill tube leak. Incidentally it would stop when the tank level reached between 1/3 and 2/3 because that was the point where the tank water was lower than the fill tube. Turns out on our unit, the fill tube enters the tank a little over halfway up the side of the tank, causing the tube to always hold some water as well when the tank is full.

I know this was a wordy reply but I was trying to explain what I have been looking at with my trailer. Your system may work the same way.
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Old 06-18-2022, 07:29 AM   #3
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On a number of occasions I have filled our freshwater tank until the light is somewhere between flickering and steady on the full mark of the control panel. But upon arriving at the next location we always seem to between 1/3 and 2/3 full - with no apparent leaks in the camper. Water pump is off and faucets are off. Are we really losing that much water from the overflow valve due to sloshing around as we drive? I guess I had envisioned baffles in the tank, but more likely the cheaper nature of our rigs means no baffles are present.

If I am losing that much on the road, would it be okay for me to glue up a valve at the bottom of the pvc drain tube? Seems logical I’d have it open literally all the time “except” when driving on the road. (Need that checklist reminder to be in all caps!)

What does everyone think? Is the overflow drain my likely culprit? Is a valve the way to go?
My guess: the problem is your reliance on “the light”. There are hundreds of reports of these things being incorrect.
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Old 06-18-2022, 09:04 AM   #4
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As mentioned, the lights, are iffy at best.
Look under your trailer, if there is a hose,, like mine,,, next to the drain hose, it is the overflow hose.
I fill my tank full when parked seasonal, then drain to what i need for the road, then fill when set up again. Incase of an outage or water problem at the park, i have a full tank for needs.
If i fill the fresh tank with a high water flow, when full, it will come out the fill port and the vent next to the fill port, then the over flow tube till it levels off.
If i fill with low psi, i fill till it comes out the overflow tube by the drain tube, then drain some out for expantion.
My lights, may still not say full, but i know its full, being set up and levelish.
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Old 06-18-2022, 10:36 AM   #5
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The factory tank level indicator LED's are triggered when water touches them and completes the circuit between bottom sensor/contact and sensor at different levels.

When filling it's logical that water may be moving in waves or even splashing so the lights aren't really registering the true level. It's been demonstrated that even having the tank wall between sensors wet can trigger an LED.

If you want a full tank just fill until water runs out the overflow. Good chance what's happening is the light flickers from "splash" and later the tank level indicators are more accurate, if you consider measuring in "thirds" accurate in a tank that may only be 6"-9" in height. With sensors mounted on one end even a slight amount or lean to one side can make a huge difference in tank level at the sensors .

For me, water flowing from fill port means full and the only time I heed the LED indicators is when it starts to show 1/3 tank. That just means I'm down to my last 10 gallons or so.
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Old 06-18-2022, 12:02 PM   #6
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Do a search for fresh tank siphoning and you will find this happens to many folks. Is it siphoning or sloshing, who knows, but it does occur.
So make sure your tank is full as others have suggested and then if you are still loosing water while traveling, then there are many 'fixes' out there ranging from rerouting vent lines to adding valves.
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Old 06-18-2022, 09:19 PM   #7
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I have noticed the same thing with my coach. I have the hose connection fill and I would fill it up until it overflowed then shut it off. When I get to my destination it would be about 1/2 full. I understand the concept that it can leak from sloshing although it's hard to believe 15 gallons would slosh out of that 1/4" hose in a four hour trip, but I can't think of any other explanation.
I do have a question on filling my tank..Since I fill it with a hose connected to my city water at home with a 30 PSI regulator on the hose, what is the likelihood that I could burst the tank? Like an earlier person said, I have on occasion been filling it and got distracted and then noticed it was coming out the overflow and I'm thinking it could have been like that for 5-10 minutes. Is there a risk of ruining the tank? I'm sure the water is going in at like 5GPM and there's only 1/4" hose for the overflow so I would think the tank must get pressurized.

What is the best way to fill these?
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Old 06-18-2022, 09:48 PM   #8
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What is the best way to fill these?
If you have a gravity fill tank and you're worried about overfilling, slow down the flow into the tank.
Unless someone has altered the tank, the excess should come out in the overflow.
People have put temporary caps on the overflow while traveling to keep siphoning from happening as you drive down the road. However, if you forget to take the cap off, you can end up with a swollen or burst fresh water tank.
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Old 06-18-2022, 11:20 PM   #9
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Best way to fill any fresh water tank is slowly Then the chances of a giant burp, or input overpowering the vent output is lessened.....
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Old 06-19-2022, 07:52 PM   #10
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Okay, seems the general consensus is (1) the indicators lights can be very inaccurate and (2) the problem of “siphoning” or loss of water while driving is a real issue.

So I can solve the first issue by just using the overflow valve as my full indicator. When it overflows…then it is full.

The second issue though. I wonder if there is a way to both install a valve that can limit the loss while driving while also allow for expansion and overflow. Perhaps a rubber pinch valve can be installed that would do a little of each?

Perhaps a self-sealing rubber valve? I’m thinking one of these maybe: https://www.aliexpress.com/item/3256...apt=4itemAdapt
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Old 06-20-2022, 07:51 AM   #11
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If you install a T fitting where the overflow comes out of the tank and route the open end of the T high enough that you aren’t worried about water coming out the top when you overfill you can break the siphon effect.
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Old 06-20-2022, 08:07 AM   #12
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Similar situation happened to me in our first year of ownership. We were boondocking in CO and stopped at a campground to refill the freshwater tank before our next dry camping spot. Filled the tank to overflowing. Got to the next dry spot and the tank was less than 1/3 full. My guess was a siphoning effect initiated by the bouncing on the roads. The bottom of the overflow tube is well below the bottom of the tank. I went to Home Depot and purchased a simple quarter-turn valve and installed it in the overflow line. Now when I fill to overflowing, I close the valve before hitting the road, then open it once I've reached my destination. I made this part of my checklist so I wouldn't forget to open it.
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Old 08-16-2022, 08:14 AM   #13
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Revisited this issue again. We had a long weekend planned with the first night boondocking followed by two nights with full hookups. For testing purposes, I filled the freshwater tank until the "F" light was on steady when I hit the button. Looked underneath and we did have a little burping out of the overflow so I knew it must be pretty full. Hooked to the truck and rolled out for the weekend.

We drove 95 miles on the first leg to our harvest host. We did hit traffic about 70 miles in, so there was a few miles of stop/go which likely had the water sloshing a lot. The host location was a nice flat paved lot. I checked water there and we were already showing only 2/3 full. Peeked underneath and the overflow hose seemed dry and the belly wasn't wet from water splashing up. Hmm.

We spent the evening in a brewery (gotta love Harvest Host!), used the toilet/sink 2 or 3 times, and then headed out at first light.

This leg was about 140 miles of highway but ended with a fairly steep uphill stretch near the end (3-4 miles of 18% grade) to reach the park. Once we parked I checked the level again - only 1/3 full. Not even a flicker of 2/3. It is possible the steep hill had some burping out, but even over the span of the 5 minutes up that hill I can't see that much pouring out. I mean it's almost a 50-gallon tank.

My conclusion is we are still losing water as we drive. Once I was hooked up to their spigots at the park I did open the tank drain to empty out for the return ride home (no stops) and it didn't take all that long to empty out. That leads me to believe the levels indicated on the panel were accurate. So time to start finding ways to keep it from leaking out.

I guess my biggest reservation is filling a tank up, closing the overflow line off with a valve, and then hitting a large bump which might make the water try to flex the tank and it not have any ability to breathe a little. This might not really be an issue (my truck has a 48 gallon gas tank!), but without seeing the tank firsthand to see how solid it is or the shape or where the mounts are it still makes me nervous. Sealed belly so I don't feel like cracking that open to look around.

That has me back on the line of trying to come up with a way of sealing the overflow with a rubber valve that can still allow for some expansion as we hit bumps and as the water comes up to temperature when the camper sits in the sun.
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Old 08-16-2022, 08:39 AM   #14
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I believe you are over thinking this common problem.
To ease your mind, gain access to to the fresh tank and its fill/vent lines by removing the bottom covering (probably coroplast). Then you can tell if you need to strengthen tank support and reroute vent lines if necessary.
Many of us use a quarter turn shutoff valve on the vent line with no problem to solve this problem to our satisfaction. I have used one on the last three RVs over many years.
Other's have success rerouting the vent line to prevent water loss.
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Old 08-16-2022, 08:51 AM   #15
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Thanks!
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Old 08-16-2022, 08:51 AM   #16
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Our new 5th wheel has two vents underneath the underbelly and we have started to see the tank siphoning out as we tow or sloshing out. Just finished a 2 week trip and found I was losing almost all my water by just driving. Added two quarter turn valves when we got home.
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Old 08-16-2022, 11:20 AM   #17
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I believe you are over thinking this common problem.
To ease your mind, gain access to to the fresh tank and its fill/vent lines by removing the bottom covering (probably coroplast). Then you can tell if you need to strengthen tank support and reroute vent lines if necessary.
Many of us use a quarter turn shutoff valve on the vent line with no problem to solve this problem to our satisfaction. I have used one on the last three RVs over many years.
Other's have success rerouting the vent line to prevent water loss.
I agree, we are getting a little OCD about this. I guess that's just the way a lot of us wired. See a problem...Fix the problem.

Obviously the overflow tube must be fairly far down into the tank to allow it to siphon and after it siphons down to that point and there's an air gap that breaks the siphon. Mine was leaking for about an hour after I filled it and sitting level in my site so I blew some compressed air into the the overflow line and that broke the siphon and the leakage stopped. I gave up and put the valve on
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Old 08-16-2022, 03:54 PM   #18
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I agree, we are getting a little OCD about this. I guess that's just the way a lot of us wired. See a problem...Fix the problem.



Obviously the overflow tube must be fairly far down into the tank to allow it to siphon and after it siphons down to that point and there's an air gap that breaks the siphon. Mine was leaking for about an hour after I filled it and sitting level in my site so I blew some compressed air into the the overflow line and that broke the siphon and the leakage stopped. I gave up and put the valve on


It actually doesn’t need to be down in the tank at all, the vacuum can start collapsing the tank if there is no way for air to get into the tank (like if you put a plug into fill port, or the check valve’s cracking pressure is high enough.)
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Old 08-16-2022, 04:04 PM   #19
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It actually doesn’t need to be down in the tank at all, the vacuum can start collapsing the tank if there is no way for air to get into the tank (like if you put a plug into fill port, or the check valve’s cracking pressure is high enough.)

Get about 2’ of tubing and a 2 liter bottle.
Cut a hole in a 2 liter bottle cap. Insert some of the tubing through the cap, insert just enough that it will stay put (use caulk or hot glue to seal it if you need to.)
Fill the 2 liter full of water and put the cap (with the tube) on. Give the bottle just a little squeeze to start the siphon (or suck on the other end of the tube) and put the end of the tube lower than the bottle. Watch as it collapses the 2 liter.

If the cap wasn’t sealed, then the siphon could only go to the level of the tube, but if the tank is sealed it is a very different story.
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Old 08-16-2022, 04:58 PM   #20
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I ran into this same problem, and from what I could tell, it seems to be a very common issue. I would siphon out nearly half the tank if I had a long drive. And this wasn't an issue with the water level lights, which have been very accurate for me on the freshwater tank.

What I did was to add a U shape to the overflow valve, with a tee in the middle of the U. On the bottom of the tee I added a 1/4 turn valve. I keep it closed when traveling. I no longer have any water spillage issues.

The reason I added a U is so that if I forget to open the valve at the bottom when filling, it will still overflow. The top of the U is higher than the top of the tank.

I know my description is confusing, so here's a picture of what I did:

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