Filling Freshwater tank

[Powellnl12]

When filling and it gets to 1/3 full it overflows through the vent. If you wait a few hours you can then fill the rest of the way. Can anyone help with why it does this?

[nayther]

fill VERY slowly. You're probably trapping air because either the vent line is incorrectly routed/pinched, etc. and if you push too hard you can expand the tank and make ugly things happen. I split a corner on my toy hauler tank filling it too fast. I've heard of tanks pushing the subfloor up too. Water is an incompressible fluid and if it's trapped it will create extreme pressuer.

[Powellnl12]

Originally Posted by nayther

fill VERY slowly. You're probably trapping air because either the vent line is incorrectly routed/pinched, etc. and if you push too hard you can expand the tank and make ugly things happen. I split a corner on my toy hauler tank filling it too fast. I've heard of tanks pushing the subfloor up too. Water is an incompressible fluid and if it's trapped it will create extreme pressuer.

It happens even if you fill slowly and we have two vents

[Herk7769]

Lots of threads on this topic.

Levelness of the tank effects tank capacity and readings.

DO NOT attempt to cram more water into the tank.

The only vent is a small vent hose and the gravity fill port is much larger. Overfilling under pressure can bulge the tank in the middle causing it to "shorten" and slip off the steel ledge it sits on; dropping the tank on the ground.

It can also crack the tank; both very bad outcomes.