Water Siphoning from Full Fresh Water Tank

[TedS]

If the vent line was upsized to 1" or larger it would probably not siphon as it would never run full. The tank would still slosh spill some.

[Jlawles2]

Cate&Rob, remembering to open the valve when filling was the issue I was trying to figure out. NC would be easiest as it will always fail closed. But you have to remember to open it. I thought about suggesting a momentary on switch attached to the panel that is positioned such that when the fill valve is selected it "runs into" the switch forcing the valve open. Then you do the same with the pump and diode to force the valve open when the pump comes on. I even thought about a setup where the brake light drives the valve closed, but you have the issue that siphoning starts at fill and continues till the light comes on. If tis a cycle valve instead of NO or NC, then you still have to reverse polarity to make it open when the pump comes on.

This can become a deep deep rabbit hole.

[Cate&Rob]

This can become a deep deep rabbit hole.

LOL! And, we have been down this rabbit hole before! See https://gdrvowners.com/forum/operati...flow-revisited

Rob

[mpking]

I think you guys are *severely* overengineering this.

The pipe servers two purposes.

1. Provide a path for water to exit if the tank is overfilled.
2. Provide a path for air to enter to prevent a vacuum.

Every other camper i've interacted with has always run the vent (in a 3/8 clear hose) up to the graveity fill connector, and mount it in the vent there.
It provided for both of those requirements. Since it goes vertical almost 2 feet, the water never flows out.

Grand Design, for some reason, has decided this is not the method they will use, and runs a horizontal pipe.. They've even gone so far to put silicon over the vent (to prevent water intrusion)
(https://gdrvowners.com/forum/exterio...1013#post31013)

Picture of the gravity fill (with vent)


That being said, I found this picture online of a guy adding a water tank to his RV.

He just went vertical about 1 foot, and put a vent screen on the end.


A better solution (which someone on here has already done, but my google-fu is failing me)
Is to disconnect the PEX overflow hose, replace the fitting on the tank with an appropirate barb, a flexible (Don't need braided) hose up to the existing gravity fill port, and replace the gravity fill port with one that does not have the vent sealed up. Down side of this. When the tank is filled, the discharge is on the opposite side of the camper. Probably why Grand design did this in the first place.

A another better solution (Possibly the best), is insert a inverse water trap into the overflow line. This doesn't stop it from providing venting action, and allows water a path to traverse if the tank is overfilled.


Question. Anyone worried that there is no screen on the end of the water overflow tank as shipped from Grand Design?

[mpking]

Note: That vertical doesn't have to be much more than 1 or 2 inches. Obviously, more is better, but you just need to break the siphon. I know we don't have alot of room.

Heck, you could even put the reverse P trap at the END of the pipe as well (this might be easier)

[Cate&Rob]

mpking
Hi Mike,

Great ideas . . . except that any Grand Design product bigger than an Imagine does not have a gravity fill connection. Power fill through the water panel connection is the only option for Reflection, Solitude and Momentum.

Rob

[mpking]

mpking
Hi Mike,

Great ideas . . . except that any Grand Design product bigger than an Imagine does not have a gravity fill connection. Power fill through the water panel connection is the only option for Reflection, Solitude and Momentum.

Rob

How about the inverse P-Trap idea at the bottom.

[Yoda]

Here is what I did an it is still working well. https://gdrvowners.com/forum/operati...hile-traveling

[howson]

Here is what I did an it is still working well. https://gdrvowners.com/forum/operati...hile-traveling

Well shoot--why wasn't that thread on the Modification list? Any other of your mods that aren't on the spreadsheet, Keith? If so, please point them out so I can add them!

[TedS]

Back to basics. The action after filling the tank is just the water in the tank that is above the vent port draining out of the tank. There is no siphon. If the downward vent tube were removed from the port, water would still drain out until the water level in the tank reached the port then the water flow would stop. The vent outlet must be above the water level so that the water can not simply drain.

[Guest]

Connecting the original vent on the Imagine is already in my plans except I purchased a locking gravity fill to complete the job. My pics and details on the overflow mod is in Neils post.

The design I'm using is outlined in the link on post #8.

There is just something wrong with a gravity fill that is not lockable in this day and age.

Jim

[Yoda]

Well shoot--why wasn't that thread on the Modification list? Any other of your mods that aren't on the spreadsheet, Keith? If so, please point them out so I can add them!

No worries. I almost forgot I had posted it here. Thought I had it at the other place. The only other mods were the trailer reverse lights. Third water pump switch in bath. And my rear storage tube. oops Also Sumo springs and Steadfast. Not sure if any of these are on your list and not sure if I did a full write up on all of them.

BTW one thing that would help in searches is a link for "Threads started by user" so one could quickly look back. Never mind, found it in advance search. I put in my user name and clicked "Started posts only" Learned another new thing today.

[Yoda]

Back to basics. The action after filling the tank is just the water in the tank that is above the vent port draining out of the tank. There is no siphon. If the downward vent tube were removed from the port, water would still drain out until the water level in the tank reached the port then the water flow would stop. The vent outlet must be above the water level so that the water can not simply drain.

Ted
My tank had the vent tube run up/over/down to drain out of the trailer. At the high point is was about 2" above the top of the tank. I discovered that it would siphon well past the vent location. As the siphoning occurred the tank walls/top/bottom collapsed inward a few inches greatly increasing the flow, until suction broke the siphon. With the tank being very long and skinny the top and bottom cold move a lot. This occurred going up hills and around corners too. First time out I lost about 1/2 a tank.

Hope this helps

[TedS]

I would suspect that water contnued to flow out as the tank tended to collapse. As the water flows out without air following to replace water that flowed out, a vacuum or low pressure results causing the tank walls to be pushed, to flex inward keeping water at the vent hole and effectively reducing the tank volume. When the tank walls reach their limit of flexing inward, the remaining water in the down tube runs out and the tank will inhale air and flex back to normal. The resulting increase in tank volume will put the remaining water level below the vent port giving the impression that water kept flowing even if below the port.

Kind of like squeezing a juice box.

[ncitro]

I would suspect that water contnued to flow out as the tank tended to collapse. As the water flows out without air following to replace water that flowed out, a vacuum or low pressure results causing the tank walls to be pushed, to flex inward keeping water at the vent hole and effectively reducing the tank volume. When the tank walls reach their limit of flexing inward, the remaining water in the down tube runs out and the tank will inhale air and flex back to normal. The resulting increase in tank volume will put the remaining water level below the vent port giving the impression that water kept flowing even if below the port.

Kind of like squeezing a juice box.

This was exactly right in my case. Once extended the vent line if it wasn’t pitched enough, then the water in the line would block the air flow. The tank was so weak (it should really have baffles for being so long and such thin material) that the tank would collapse rather than draw in the water that was in the vent line. I’ve seen it collapse so bad that when I would go out with my leaf blower and blow the water in the vent line up to burp the tank, it would read 50% on my Seelevel after it returned to normal.