Dude check this out!

[steve R]

I have an 2007 Winnebago Sightseer 35J. When i fill my water tank i shut the water off when it starts to come out the over flow. I noticed that the water seems to come out of the over flow for some time before it stops, when i arrive at the campground i could be has much as a third low. I decided to investigate whey this was happening, I filled the water tank for a camping trip this weekend and the same thing happened, If i lift my bed up i can see my water tank and as the water was being siphoned out after the tank was filled the tank was actually caving in. I opened the main drain and let some air come into the tank and break the siphon. There is a elbow that comes off the top of the tank some tubing is connected to it to drain the over flow water out the bottom of the motor home. When i removed the elbow there is no other tubing that goes into the tank. I'm baffled as to how it can have such a strong siphon.

[AFChap]

Baffled or not, many do the same thing, draining a substantial amount of water from the tank while driving. Whenever I fill my tank to the point that it comes out the overflow, I ALWAYS pull the fresh water dump valve for 30 seconds or so to break the vacuum/siphon.

[RV Wizard]

You could install a siphon break on the overflow that will allow air into the line when it starts to over flow but will stop when no pressure is pushing water out.

[steve R]

Whats the best way to install a siphon break.

[PlayItForwrd]

If you're water is siphoning from the top overflow and is "caving in" the water tank, the problem is that you've filled the tank to the point there's no air space, and atmospheric pressure is now free to 'push' the water out of the tank thru the vent, and will continue to do so until the tank can support the air pressure, or the vent can find an air bubble. The simple answer is, you just need an air vent. And the easy solution is to just leave the fill cap open for awhile, so some of the water can slosh out. Water cannot siphon itself out of a vent if there's air space between the water and the vent outlet...

A harder solution is to add a second air vent, which someone did to our Allego. Both vents lines have 1/4 turn ball valves too. With 2 vents, if water comes out of one, air will come in thru the other, stopping the siphoning...

[RV Wizard]

Quote:

Originally Posted by steve R Whats the best way to install a siphon break.

The siphon break should be where the over flow port comes out of the tank and before the line that either leads to the vent on the side of a gravity fill or down to the ground.

[Cruzer]

You can also just add four 90 degree ells and some tubing to make a loop (a square, actually) in the overflow. I'tll act like a trap and will prevent draining. That's what I did on my Suncruiser.