Siphon Action Draining Water Tank - Problem Solved!

[JBill9694]

Yoda, maybe your R2D2 unit decided not to tow all that water up those mountains. LOL

[Jlawles2]

I see that the dual tube setup works, however I can also see where it's possible to get both tubes started with flow and have both of them syphon until conditions are right and the shorter tube allows air to enter. I don't remember who, but someone Tee'd the vent and ran the extra line up instead of down, thus air will always enter the vent tube.

[Cate&Rob]

Jlawles2

The single overflow line out of the tank cannot supply enough water to keep both down tubes full under vacuum draw .. . thus breaking the siphon. If the tank were pressurized and expanded, I could see where both down tubes could be kept full for a while. We did discuss taking one tube up for a better vent, but this would put a potential overflow or slosh pumping outlet inside the coroplast. Ideally the vent line should go up and exit outside the RV body.

Rob

[Bobsrv]

I purchased some hard rubber cork style stoppers from hardware store filled tank till water stared coming from over flow. Installed stoppers and arrive at next stop 200 miles of rough road Valdez to Chicken Alaska then to Dawson City Yukon . No water loss and plugs held tight.

[Country Campers]

I purchased some hard rubber cork style stoppers from hardware store filled tank till water stared coming from over flow. Installed stoppers and arrive at next stop 200 miles of rough road Valdez to Chicken Alaska then to Dawson City Yukon . No water loss and plugs held tight.

When you pulled the stoppers was there any water that came out? Just thinking if there was any pressure inside the tank that may force water out when you opened the overflow.

Brian

[Bobsrv]

No water came out . Opened after another day rough roads.

[dagraf]

Jlawles2

The single overflow line out of the tank cannot supply enough water to keep both down tubes full under vacuum draw .. . thus breaking the siphon. If the tank were pressurized and expanded, I could see where both down tubes could be kept full for a while. We did discuss taking one tube up for a better vent, but this would put a potential overflow or slosh pumping outlet inside the coroplast. Ideally the vent line should go up and exit outside the RV body.

Rob

I have a 2022 Imagine XLS 17MKE with the double-tube modification. If I gravity-fill the tank until the gauge says Full...no problem. If I keep filling until water is gushing out the fill tube, as expected water drains, from both vent tubes. It keeps draining until it gets to 2/3 full...kind of annoying but OK. If I start filling again, both tubes start draining immediately. Seems to me the siphon is getting re-established. Maybe if I had drained the tank a bit further, the water level would have dropped below the end of the tube inside the tank and break the siphon but I didn't think to try that.

Seems like I'm headed to some kind of solution involving putting a valve on one tube and running the other tube upwards.

[TedS]

It's overflow, not siphon. Gravity fill will prevent siphoning by admitting air into the tank.

[dagraf]

It's overflow, not siphon. Gravity fill will prevent siphoning by admitting air into the tank.

I guess I don’t understand why I got “overflow” when I started adding water to a 2/3 full tank.

I did these tests because I lose so much water while driving and want to understand why. I’d like to solve that problem, and from my reading these threads, it seems like what I could do is block one line and extend the other line so its outlet is above the top of the tank.

[Guest]

I have a 2022 Imagine XLS 17MKE with the double-tube modification. If I gravity-fill the tank until the gauge says Full...no problem. If I keep filling until water is gushing out the fill tube, as expected water drains, from both vent tubes. It keeps draining until it gets to 2/3 full...kind of annoying but OK. If I start filling again, both tubes start draining immediately. Seems to me the siphon is getting re-established. Maybe if I had drained the tank a bit further, the water level would have dropped below the end of the tube inside the tank and break the siphon but I didn't think to try that.

Seems like I'm headed to some kind of solution involving putting a valve on one tube and running the other tube upwards.

If you have a gravity fill, install a SharkBite valve on one overflow and run the other to the gravity fill vent that is most likely not connected.

A similar approach with the single overflow.



Jim

[Cate&Rob]

dagraf

The vent/overflow connection is at the very top of your water tank. If water is siphoning out until you reach the 2/3 level, the bottom of the tank is collapsing (being drawn upward) to lift the water up to the overflow. This has been proven to happen with a single overflow pipe, but the testing done by Grand Design confirmed that the twin overflow will break this vacuum.

How are you determining that the siphon action happens down to a 2/3 level? If the tank is collapsing (reducing in volume) to lift the water to the overflow, the water level gauge would show “full” until the siphon vacuum breaks and the tank returns to normal shape.

if you fill with a flow meter and then catch what overflows in a bucket of known volume, you will be able to get a better measure of what is going in and what is coming out.

Rob

[dagraf]

If you have a gravity fill, install a SharkBite valve on one overflow and run the other to the gravity fill vent that is most likely not connected.

A similar approach with the single overflow.



Jim

Yep, that’s what I’m thinking. Thanks for your help.

-David

[dagraf]

dagraf

How are you determining that the siphon action happens down to a 2/3 level? If the tank is collapsing (reducing in volume) to lift the water to the overflow, the water level gauge would show “full” until the siphon vacuum breaks and the tank returns to normal shape.

Rob

Well that’s scary. I had the gravity fill open, so hopefully that eliminates the possibility of having collapsed the tank.

I was using the tank level indication on the trailer’s control panel.

Thanks!

[Cate&Rob]

Well that’s scary. I had the gravity fill open, so hopefully that eliminates the possibility of having collapsed the tank.

I was using the tank level indication on the trailer’s control panel.

Thanks!

If the tank was not collapsing . . . how did a 2/3 level get up to the vent/overflow at the top of the tank? Perhaps trusting the tank level indicator is part of the problem. They are notorious for being inaccurate.

Rob

[Jlawles2]

I can see that the overflow siphoning happened until it broke the suction. Once this happened and the tank relaxed, the gauge could then read 2/3 full.

If the top and / or the bottom are collapsing inward during the siphon, it's possible that it is cold enough for the tank to hold shape (stretch here hoping you are somewhere that it's cold. Then once you start filling since the top is already deformed, it starts pushing water out the overflow.