05/12/2026
From the "I learned something the hard way" files... While building my ambulance to RV conversion, where I learned not only the necessity of having a vent to let air escape from the tank while filling, but also to let air in when using water out of the tank.
While filling, the tank will/can pressurize to the point of expanding or even breaking - and while drawing water out, the negative pressure can possibly collapse the tank - and ALL of this puts excess pressure on your entire plumbing system, lines, valves, pumps, etc.
Another thing I learned, is that if you have TWO tanks - I didn't think I'd need a air relief vent on BOTH tanks, but after test filling my tanks while I could still see both tanks at the same time during the conversion, indeed you DO NEED suitable relief air vents on BOTH tanks, as the water in the line connecting them creates two separate air chambers, and the same negative effects happen on the UNVENTED tank.
The other interesting thing I learned, which you hopefully won't ever encounter... I vented the tank with an outlet and elbow hose that was routed to the outside of the RV (below the tank) but only raised the outlet about a 1/2" above the top of the tank, figuring that was plenty of height above the water level - however, while filling the tank fully the 1st time, after the tanks were built in and NOT visible easily, I was mystified as to why water started running out of one of the tanks vent tubes once the tank got full - which I THOUGHT made sense, and wasn't an issue... but it WAS an issue! A siphon effect had started, and there was no way to break the siphon (tried putting my finger over the vent tube where water was being drained multiple time, and for several minutes) - and the entire 100 gallons siphoned out. I had to open up the the access to the tank, and raise the height of the vent tube exit about 10" above the tank top, and change the vent tube routing to solve the siphon issue.
Learning new things all the time.
