What is this hose for?

[Watsons]

Hi all. This is on a 2017 339 FLQS.
The mystery hose in the picture just dribbles a slight amount of water when the tank fills. The smaller hose lets me know the tank is full with full flow coming from it and of course the other one with the valve is the tank drain.
What's the purpose of the larger one with the flimsy rubber end?
Oh...by the way...we picked up the unit a few weeks ago and can't be happier with the floorplan, storage is adequate and build quality seems ok so far.
Will start another thread outlining our experiences to date..
Cheers

[Fire_Instructor]

That’s your tank overflow.

[GHen]

The flimsy one is my fresh water tank overflow on my 291rlts.

[Jim Smyth]

Thats also the hose that will siphon and drain your water tank. I twist it up and put a clamp on it when it starts doing that with a full tank.

[Watsons]

Interesting. it seems my tank overflow is the smaller one that is open ended. Its the one that indicates a full tank when filling..If the larger one is suppose to be the tank overflow then what is the smaller one suppose to be for?
Cheers

[JohnWedell]

The smaller one is the air vent for the tank. The large one is the overflow.

[REWahoo]

Originally Posted by Jim Smyth

Thats also the hose that will siphon and drain your water tank. I twist it up and put a clamp on it when it starts doing that with a full tank.

^ What he said. I also clamp the overflow shut to prevent losing water while on the road.

Originally Posted by JohnWedell

The smaller one is the air vent for the tank. The large one is the overflow.

^ Yep.

[Watsons]

Great, tank must be plumbed wrong but in reality it still fills so will live with it.
I think I'm going to plug the smaller one and see how much more water I can stuff into the tank before it comes out of the larger one. Might just leave the small one plugged.
Question though.
Why would Jayco plumb in both an overflow and tank vent? Wouldn't just a tank vent/overflow combo do the same thing but better? By having only one overflow/vent, if it started siphoning it would create a partial vacuum in tank which would prevent the siphoning??
Thoughts?

[Jim Smyth]

Its definately a bad design. Ita also a known problem. You have to deal with it or rip the underbelly apart to make it right.

[vcbice]

I really don't know why they put a vent line and an overflow on them. When mine gets full both 'leak' water

I've never experienced the siphon effect so have not clamped the overflow line.