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Old 08-28-2014, 12:10 PM   #1
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Tank vent tube drains fresh water tank problem

I have a 2011 Sightseer 33C

When I fill the freshwater tank using the recommended procedure, the tank will fill and then water will start coming out the tank vent tube located beneath the coach. It will continue to flow out this tube and almost drain the tank. Today, I measured the water and over 65 gallons flowed out before it stopped.

Why is this happening?

The tank vent tube is only connected at the top of the fresh water tank. If the tank is overfilled, as stated in the manual, it would only drain the water out until the level is below the tank vent tube. Why does it drain the entire tank? I can't see how it could siphon it out.

Any and all help is appreciated.

Thanks
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Old 08-28-2014, 12:22 PM   #2
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The overflow tube has been installed incorrectly.

Once you find it I would change the way it is configured so that it doesn't siphon most of your fresh water onto the ground.

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Old 08-28-2014, 12:30 PM   #3
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Quote:
Originally Posted by Laker44 View Post
I have a 2011 Sightseer 33C



The tank vent tube is only connected at the top of the fresh water tank. If the tank is overfilled, as stated in the manual, it would only drain the water out until the level is below the tank vent tube.

Any and all help is appreciated.

Thanks
IMHO a siphon only works if the "wet" side stays immersed and the outlet side is below the level of the fluid. Make sure the tank connection is actually terminated at the top not extended into the tank and that the outlet is above the top of the tank. let us know we love a mystery.
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Old 08-28-2014, 01:28 PM   #4
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The vent/overflow tube fitting is spin welded to the top of the tank and you would think a siphon could not occur, but as you say a lot of water can come out.

I think what happens is that the tank expands a lot as it fills because the air can't get out fast enough. When you stop filling the tank it starts to go back to it's original size and pushes water out. It will continue to flow even after the tank has relaxed because the top of the tank is sucked down keeping the vent tube at the level of the water and siphoning water out. It will continue to siphon until the vacuum on the tank and the outside air pressure equalize.
That is my best guess anyway. I know if you open the gravity fill to increase the venting available and fill very slowly the amount of loss can be decreased a lot.
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Old 08-28-2014, 02:54 PM   #5
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Laker44, does you drain pipe looks like this? Note the larger pipe fastened around the shorted interpipe to break the vacuum. I think this was a fix sometime ago. I could not fine the older post about this same problem.
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Old 08-28-2014, 06:35 PM   #6
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Laker44, does you drain pipe looks like this? Note the larger pipe fastened around the shorted interpipe to break the vacuum. I think this was a fix sometime ago. I could not fine the older post about this same problem.
Yes, that is correct configuration. However the water is draining out of the vent tube, not the drain. The drain value is closed. Opening and the closing the drain valve had little effect other then draining more water out of the tank.
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Old 08-28-2014, 06:58 PM   #7
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Sorry, meant the vent tube it is behind the drain pipe.
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Old 08-28-2014, 07:52 PM   #8
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The easy fix for this is to take a small drill and make a hole in the top of the over flow hose. Then a siphon can't get started.
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Old 08-28-2014, 07:55 PM   #9
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Looks to me like the vent goes down into the tank and the pipe over the vent is a vacuum breaker. If it is not open at that slip over joint it will siphon the tank.
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Old 08-28-2014, 08:18 PM   #10
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TonyDi and I were posting at the same time. Tony is dead right, that will stop it, hard telling how long that inner pipe is on the outside of the tank. Really an odd set-up, some one other than a plumber designed that.
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Old 08-28-2014, 08:19 PM   #11
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Pretty common issue here is a simple fix turn you water pump on and run some water This will cause it to suck from the tank and break the siphon. I have seen many tanks that would nearly suck themselves flat from the siphon but if you run the pump and pump a few gallons out it stops the siphon every time.
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Old 08-28-2014, 08:21 PM   #12
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You can also open your drain for a few seconds to stop the siphon.
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Old 08-29-2014, 06:02 AM   #13
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I just run the outdoor shower that is located in the wet bay for a few seconds. That also breaks the vacuum/siphon.
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Old 08-29-2014, 06:10 AM   #14
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Saw this talked about on another forum. I think the best solution I saw was this one below.

My tank use to have the same siphon problem. Where my vent exits the top of the fresh water tank it makes a 90 with a short piece of tubing to the edge of the tank where there is second 90 and routes the overflow tube down and out the bottom of the compartment to the outside. At the second 90 I removed it and replaced it with a Tee, reattached the down tube and put a 6" piece of tubing in the upper end of the tee leaving it open on the top. Now I can fill the tank to the point it runs out the over flow but it will not siphon. Worked for me on the last two motorhomes.
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Old 08-29-2014, 06:49 AM   #15
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Originally Posted by grtharris View Post
Laker44, does you drain pipe looks like this? Note the larger pipe fastened around the shorted interpipe to break the vacuum. I think this was a fix sometime ago. I could not fine the older post about this same problem.
Thanks. I do not have the larger outer pipe on the tank vent. It is a 1/2" tube all the way out to the bottom of the coach.

Thanks
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Old 08-29-2014, 08:35 AM   #16
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Vent Tube

Laker44, here is a better picture with dimensions should have send this one first.
TU-9 | 70 = 1/2 pex 70mm long
TU-15 | 545 = 3/4" pex 545mm long
Here is the link to your plumbing drawings and more info:
http://www.winnebagoind.com/diagram/...d33c_plumb.pdf
http://www.winnebagoind.com/diagram/...g_callouts.pdf
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Old 08-29-2014, 08:47 AM   #17
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Thanks. Very helpful
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Old 08-31-2014, 11:01 AM   #18
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I installed a valve in mine. My problem, aside from the siphon, is that my driveway is very steep. As I drove up my driveway with a full water tank, the water shifts to the rear and would pour out the overflow.

Be sure to open the valve when you fill the tank if you do this.
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Old 08-31-2014, 11:12 AM   #19
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I installed a valve in mine. My problem, aside from the siphon, is that my driveway is very steep. As I drove up my driveway with a full water tank, the water shifts to the rear and would pour out the overflow.

Be sure to open the valve when you fill the tank if you do this.
With the vent shut off your also making your water pump work hard to draw water out of the tank it needs the vent. Without a vent the only way it can get water is to suck the tank flat.
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Old 08-31-2014, 11:26 AM   #20
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With the vent shut off your also making your water pump work hard to draw water out of the tank it needs the vent. Without a vent the only way it can get water is to suck the tank flat.
I have been using it this way for 6 years and my tank seems OK. If I were concerned, I could open the valve when I got to my destination.

I guess that I think of that line as an overflow rather than a vent.

The tank is also vents through the right side fill, it isn't sealed all that well.
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