Vent tube modification for FW tank

On my tank, the vent line is attached at the top of the tank, near the gravity fill hose, and does not extend into/inside the tank. The other end of that vent line is actually attached next to the cap of the gravity fill port, above the tank level.

I don't understand how this "siphon" is happening. If it is siphoning 1/4 or 1/3 of the tank volume, then that would require the vent line to be down inside the tank 1/4 or 1/3 of the way, and the loose end to be below the tank, regardless of how high the line goes (or loops) between the two. Can someone post a diagram showing how their tank lines are set up, including both the vent line and the fill line(s)?

UPDATE: The vacuum break did not work...water spit back out at me. I did install correctly, but it is lower than the tank. I am going to move the vent a bit higher than the tank to avoid this...as suggested here.

beemerphile1 wrote:
Any chance you can add some tubing and put a loop up higher? I got the idea from the vented gas caps on dirt bikes. The vent tube would go straight up from the cap and then back down at the forks. Might stop the splashing and siphoning.

This is what I did to stop my water from siphoning . I raised my over flow tube 6 ins above the tank and held it up with a clip to form a loop in the tube. It was an easy no cost fix. I would not put a plug in the tube as some have suggested, I saw a rver up in Alaska break the supports that hold up his tank because he forgot to remove his plug before filling and his tank expanded.

You really don't want to stop water from flowing out of the vent. If you forget toturn the handle or if it is automatic and water starts overfilling the tank you want a place for it to go. Otherwise you have a swelling tank that may bust the mounting straps or burst and so on. All not good things.
I've accidentally overfilled the tank at the water hose connection and seen a hard discharge from the vent.
It is better to let a little run out, of the gravity feed and be down a couple gallons from absolute full.
chances are if that full you may have some come out the vent anyway. better that than the consequencies of over filling.
We use the gauge inside the coach. There is enough room if it starts to show full to go out and cut off the hose.
They should and may, on some models, put a set of gauges, for all tanks in the wet bay. A couple of wires for each gauge and the gauges seems so simple. if I can find someone to do it I am.

On ours, I just drilled a 1/16" hole in the overflow tube at its highest point where it connects into the tank. As the tank overflows in while filling, only a drop or two come out the drilled hole. Then, as soon as I stop filling, the drilled hole becomes the vacuum break.

Cheap, easy, and fast.

I had an rv that had the fill/vent points too close to the top of the tank and it lost a lot of water out the vent on curvey roads. I got a longer tube and made an inverted U and no more problem. Craig

dalenoel wrote:
I had the same problem with my Jayco and joined the two overflow lines into one, ran a line to the rear of the TT so I can open when filling and then shut of the valve when full. Stops lose of water after full and also bugs from entering FW tank.

Shut off valve and then Joined Lines


Photos are not posting

X2 use mine to fill the dogs water bowl until it stops, then just leave it open. Had the upward U over the propane tanks but once the siphon starts that did no good.

I had the same problem with my Jayco and joined the two overflow lines into one, ran a line to the rear of the TT so I can open when filling and then shut of the valve when full. Stops lose of water after full and also bugs from entering FW tank.

Shut off valve and then Joined Lines


Photos are not posting

I will try to find the pictures that were posted! The original vacuum breaker is utilized but is considerably higher than the original location!

Any chance you can add some tubing and put a loop up higher? I got the idea from the vented gas caps on dirt bikes. The vent tube would go straight up from the cap and then back down at the forks. Might stop the splashing and siphoning.