Tank vent tube drains the entire tank problem

[Liltippy]

I have a 2011 Winnebago 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

 

[blw2]

there has to be a way for the tube to pick up air, in order for the siphon to break.
I suspect that the tube does dip down into the tank
otherwise there is pressure continuing to push into the tank to displace the water up and out.
these are the only two tings I can imagine.

Don't fill the tank to the point of overflow, and I'll bet it won't siphon out.....

If that's not the case, and it does siphon out even if the tank is only partially filled (or filled but not to the point of overflow) then my bet is on something continuing to pessurize the tank......

What is getting into the tank to displace the water that is siphoning out?

 

[Clay L]

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 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.

 

[blw2]

could there be a pressure relief valve taht is sticking open, and you are mistaking it for the vent tube?

 

[John Canfield]

Any similarity with this situation?

 

[howdymi]

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.

 

[Liltippy]

Any similarity with this situation?

Yes, that is the same.

thanks to all of you for your ideas and suggestions.  Lots to think about.

 

[Gary RV_Wizard]

It appears the vent line goes too deep into the tank - it should be near-flush with the top so the vacuum (suction) breaks after a bit of water flows out. If you can't get at the vent line to raise it up, install the Tee vent as  howdymi described. Works like a charm!

You may see a suggestion to add a valve in the vent line, but that is almost sure to be troublesome. If you forget to open it while filling, your tank fills slowly or not at all, plus you could cause damage by pressurizing the tank. Last, you may not be able to pump all the water from the tank. And you WILL forget sometimes.