Problem Filling Fresh Water Tank

[Team Wilhelm]

Help. When filling my fresh water tank it should flow out the overflow vent once filled right. It would always do this. Recently when I filled it now it starts to expand the tank and practically lift it off it’s rest. Water does not flow out vent hose. I tried blowing into the tank thru the vent hose and it feels not plugged. I felt no restrictions. What could be a matter ? As a note my vent hose comes out of tank at the top on the passenger side end and then goes up about 12” around some parts of RV before coming back down thru a hole in floor. Thanks…Ron

[Joker]

Things I can think of:
1. What you think is the vent hose really isn't and there's a plug in the real one.
2. There's both a disconnect in the vent hose AND a plug, with the plug being on the tank side and the disconnect beyond the plug.
3. The vent hose connection is on a part of the tank that also gets lifted up as it fills, so the water level isn't making it to the overflow point.
4. A more likely cause, given the circuitous vent hose path you describe, is that the hose travels too high relative to the top of the tank before descending. Since that overflow hose is not fed by pressure but by gravity, you wouldn't see flow until the top of the overfilled tank got above the top of the vent hose. Then it would drain and probably siphon itself down to the normal water level. That's what I would check first.

[Herk7769]

Could also be a bug built a nest in your vent hose.

[Team Wilhelm]

Thanks Joker. It was working ok till about 3 fills ago this year. Where I would normally see water coming out when it was filled not does not. I'm thinking per your #4 comment that they raise the hose a bit so when driving it don't pour out. I'll follow the lines again.

[Superchief]

I would believe you have too much air in the tank. Do the facets spit a lot of air when you turn them on? Water always seeks its own level, no matter the circumstance, if the vent and or fill hose has been sagging of its own weight, it might have finally got to the point where part of it dropped below the highest part of the storage tank thereby creating a “trap” like you have under your sink.

[Herk7769]

Best photos I have of the vent hose and why it spits when you fill it and siphons water out as you drive. The photos show why you also can't get a full tank of water if you fill at a water filling station while you are hooked up.

The last photo is not my camper but shows the effect of an out of level fill attempt.