Dispelling a fresh water tank myth. Vacuum

[m2bandit]

I was blessed to have a fresh water tank setup that will drain 40 gallons of my 80 gallon setup before it stops siphoning. Extra long overflow hoses I guess.

When I first started dealing with this issue I saw a lot of people say not to put valves on them because it will create a vacuum when you use water and damage the tank. Or it would over-pressurize and somehow burst the tank instead of just pushing your hose out.
Your fresh inlet is vented so any vacuum pressure would be filled through there.
I do open them during filling otherwise they may not equalize properly and I also wouldn't know the tanks full until it flows out the inlet.


But here's the proof. After 3 days of showering and doing dishes If I open my overflows they still siphon out water. If there was a vacuum it would interrupt the siphon affect.

[JimD]

I know your pain. My Greyhawk would drain 1/3 of my tank while I was enroute to my destination. I put valves on all 3 tank hoses which solved that problem, sort of.

I discovered that if I filled the tank fast and the siphon started, it would continue to drain the tank way down. The secret to my fill was to slow down at the end and stop when it started to trickle out before it started to siphon everything. Then I would shut my valves until I got to my destination.

Well, my valves are located below my slide which is a bit inconvenient to get to and I, on more than one occasion, forgot to open any valve. The tank did indeed get a suction inside which was evident when you finally opened the valve and could hear air rushing in. Didn't seem to hurt anything or lower water flow inside. But I did punish myself (no beer for a night) so I would not forget to open the valves again. If I wasn't so forgetful that could have worked but....

My latest "fix" was to extend all 3 tank hoses up into my outside shower/water fill cabinet and install some nice valves there. Very easy to get to. No way (I hope) to forget to open the valves when filling. It appears to also prevent a siphon from starting as the ends are above tank level. When I filled the tank water came out all 3 hoses so it is fuller than any other time. Sine I dry camp and only have 40 gals without refilling I want every drop I can have.

In theory when water is drawn from the tank the vent hoses should get the water sucked from them back into the tank and then allow air in. I am in the initial testing of this new set up but unfortunately am staying at some full hookup sites so I can't really test the new valve setup.

I think the secret is getting the vent hoses above the top of the tank. Kind of hard to do considering the tank is mounted up against the floor. That is why I routed mine up into the shower/water fill area which is above the tank plus can get as wet as it wants.

[m2bandit]

Originally Posted by JimD

I think the secret is getting the vent hoses above the top of the tank. Kind of hard to do considering the tank is mounted up against the floor. That is why I routed mine up into the shower/water fill area which is above the tank plus can get as wet as it wants.

From what I've read, the official jayco solution is putting a loop in it idealy higher than the tank.

That would be through my living room floor. This is simple enough without making me remove coroplast again.

[Rustynuts]

It all depends on the setup you have on your RV. Some have use a gravity fill, some have a pressure fill which is the case on our Seneca. If you put a valve on the vent tubes and have it closed there is no way for air to vent into the tank while using it and it will collapse the tank. In the case of a gravity fill, then it can vent through he filler cap. I fixed mine on our Seneca by pulling the vent lines up and over the top of the tank and plumbed them into a pvc pipe manifold that provides an air gap up above the tank level. It has permanently fixed the issue.

[RichR]

Our 2018 21QB has two 40 gallon tanks and both tank overflows start to run as soon as the tank is full. I carefully watch for when they start to flow when filling and shut the water supply off then I go inside and pump a small quantity of water from a cold water faucet. I believe this causes a enough suction on the tanks to stop the siphon action because the water stops flowing from the tubes shortly after I do that.
I believe that tubes are improperly attached to the tanks or the tubes are inserted too far into the tanks from the top.

[Williasi]

I am a new rv owner. I have a 2020 Class C - Entegra/Odyssey - How can I determine if my is a pressure fill or a gravity fill. I need to do something. I found out about the siphoning the hard way when I had no water at the campsite. It seems the Jaayco should supply a fix or supported work around. The hose extension with the PVC manifold above the tank level seems the best approach. Need to see how much room I have to work with. Disappointed in the product.

[TaftCoach]

Originally Posted by RichR

Our 2018 21QB has two 40 gallon tanks and both tank overflows start to run as soon as the tank is full. I carefully watch for when they start to flow when filling and shut the water supply off then I go inside and pump a small quantity of water from a cold water faucet. I believe this causes a enough suction on the tanks to stop the siphon action because the water stops flowing from the tubes shortly after I do that.
I believe that tubes are improperly attached to the tanks or the tubes are inserted too far into the tanks from the top.

Both tank overflows? Same model, I only have one overflow for both tanks.

[JFlightRisk]

Originally Posted by RichR

Our 2018 21QB has two 40 gallon tanks and both tank overflows start to run as soon as the tank is full. I carefully watch for when they start to flow when filling and shut the water supply off then I go inside and pump a small quantity of water from a cold water faucet. I believe this causes a enough suction on the tanks to stop the siphon action because the water stops flowing from the tubes shortly after I do that.
I believe that tubes are improperly attached to the tanks or the tubes are inserted too far into the tanks from the top.

We have the predecessor to your 21QB, also with the dual FW tanks. They are very shallow, which exacerbates the problem, but there is no tubing on the inside of the tanks. The vents and overflow tubes attach to nipples that are spun-welded to the upper side of the tanks. I fixed the issue by attaching 1/4 turn valves to the ends of the overflows. I open them to fill, close to move it, then open them again to use the pump.