Siphon Action Draining Water Tank - Problem Solved!

[Yoda]

Keith,

I have a valve installed and it made no difference at all. I could stop the flow with the valve, but even after more than half the tank was gone, the flow would start right back up as soon as I opened the valve. I believe that no air was getting into the system and the flimsy water tank was collapsing. It's never been this bad before.

Jim

Other than dropping the belly I would suggest something like I installed. It's been working perfectly. You don't need the air vac I used. Just cover the pipe end with a screen. Even with the new design there will still be some surge loss while driving. My set up prevents even that. Hope this helps
Keith

[Canyonlight]

I'm can't wait any more. I need to drop the underbelly and apply Rob's fix. A couple of weeks ago, I filled my fresh tank and let it go too far. It started to spill out of the overflow. I could not get it to break the siphon no matter what I tried. I pressed the input valve to let air in, and there was no stopping it. I ran a few gallons of water out of one of the faucets, and the overflow kept spilling out. I opened the drain valve which did nothing more than start draining the water out of both the overflow and the drain valve at the same time. Water was still coming out of the overflow tube when I was at only 1/3 of a tank. The only way I could get the flow to stop was to use my compressor to pump air into the overflow! This has to be terrible on the fresh water tank. Now, I just need some cool weather here in Tucson before tackling this project.

Jim

Jim - a quick and easy solution......get a wine bottle cork from Andi (or other appropriate size stopper) and stuff it in the overflow tube ! I'm sure Andi has one (or maybe two lol) to spare ! If not, I'll have Carol send some your way.

Dan

[Andi]

Jim - a quick and easy solution......get a wine bottle cork from Andi (or other appropriate size stopper) and stuff it in the overflow tube ! I'm sure Andi has one (or maybe two lol) to spare ! If not, I'll have Carol send some your way.

Dan

Thanks for the laugh this morning, Dan! 🤣🤣. But I’m not sure where you got the idea that I might have wine corks 😉

[ThePowells]

I would imagine that even though it is inside of insulation that the warmer that it gets the more flexible the tank becomes.

[Bobsrv]

My 295RL has the over flow and vent tunes shown in post 51. I can leave the campground with full tank and by the end of a 200 mile drive I show 1/3 tank of water. It’s constantly loosing 2/3 of water. I shut off water when vent overflow starts dripping let it rest for a few minutes and top off..I’ve checked under trailer for sagging ; all looks good and passes test of pushing on underbelly. Any suggestions ?

[Country Campers]

My 295RL has the over flow and vent tunes shown in post 51. I can leave the campground with full tank and by the end of a 200 mile drive I show 1/3 tank of water. It’s constantly loosing 2/3 of water. I shut off water when vent overflow starts dripping let it rest for a few minutes and top off..I’ve checked under trailer for sagging ; all looks good and passes test of pushing on underbelly. Any suggestions ?

Only fill the tank until the gage says its full. This will leave an air gap at the top of the tank and not let the water start the syphon action from filling, also recommended by GD to only fill to the indicated full level.

Brian

[Guest]

The mod in the link below solves all issues with siphon, slosh and tank collapse. A high flow (to match forced fill) single vent high on the side of the rig with no overflow would be ideal during manufacturing as a permanent fix to these issues.

https://gdrvowners.com/forum/operati...ent-water-loss


Jim

[Cate&Rob]

My 295RL has the over flow and vent tunes shown in post 51. I can leave the campground with full tank and by the end of a 200 mile drive I show 1/3 tank of water. It’s constantly loosing 2/3 of water. I shut off water when vent overflow starts dripping let it rest for a few minutes and top off..I’ve checked under trailer for sagging ; all looks good and passes test of pushing on underbelly. Any suggestions ?

You are the first to report that the twin overflow “doesn’t work”. Grand Design did bench testing to confirm that the second tube would break the siphon in the first . . . before implementing this design across all their product lines. It was never claimed that this design would stop “slosh pumping” out the overflow, but if this sets up a siphon, the water will stop flowing.

If you drive a particularly rough or twisting route, I can see how this continual motion could pump a lot of water out the overflow/vent lines. Raising the overflow to considerably higher than the tank (as described in recent posts on this thread) or capping the existing overflow(s) while travelling would be your only alternatives.

Rob

[Jlawles2]

Bobsrv You could try clipping the shorter line as close to the coroplast as reasonable as this will help increase the imbalance needed for the system to work. The issue I see is that if both lines get full of water, the siphoning will continue. The different length lines in theory should stop the siphon, however, it's not a 100% guarantee. Something has to create an imbalance in the 2 lines (one being shorter showed to be enough) which will result in reverse flows of the 2 lines.

[Cate&Rob]

Jlawles2
Hi Joseph,

You are absolutely correct that a difference in tube length would be the best configuration . . . for the reasons you describe. Grand Design missed this part of my recommendation and bench tested with tubes of more-or-less the same length. The concept still worked. I think this is in part because one tube feeds two tubes at the highest point. The one vent tube coming out of the tank can’t supply enough water to fill both down tubes and this helps to draw air in through one of the tubes and break the siphon.

Rob

[TedS]

Why not use one larger single tube for the downward vent if two tubes of the same diameter are more than the tank overflow can fill. Use one 3/4 inch or 1 inch tube, as water partially fills the tube and flows down, air can flow up. Anybody ever try that? Like all downward vents it won't stop slosh spill.

[Cate&Rob]

Why not use one larger single tube for the downward vent if two tubes of the same diameter are more than the tank overflow can fill. Use one 3/4 inch or 1 inch tube, as water partially fills the tube and flows down, air can flow up. Anybody ever try that? Like all downward vents it won't stop slosh spill.

Hi Ted,
That idea was definitely discussed (and I think it would work) but it was important to Grand Design that the proposed solution be constructed from easily available "parts on hand". Sourcing a connector to go from the standard 3/4" Pex drain tube to a larger diameter plus adding different size tube and clamps than used anywhere else in the assembly plant . . . was not their preferred solution.

Rob

[TedS]

Cate&Rob interesting. They already use large tube for gravity fill, didn't need to be pressure tube.

[Bobsrv]

I filled my tank to 2/3 full before leaving Valdez drove 120 mile and arrived with 1/3 tank. I’m sure water is draining out on all hills we climb. On a slight grade on a view point turn out II had a a-trail of water from over flow tube.

[Cate&Rob]

I filled my tank to 2/3 full before leaving Valdez drove 120 mile and arrived with 1/3 tank. I’m sure water is draining out on all hills we climb. On a slight grade on a view point turn out II had a a-trail of water from over flow tube.

When you completely fill the tank to overflowing and stop filling, does the overflow stop or does it set up a siphon action, drawing in the bottom of the tank and continuing to empty the tank? This is the situation that the twin overflow is designed to stop.

Sloshing water out the overflow while driving is a different problem that can only be remedied by closing the overflow while moving and perhaps installing a higher vent location as previously discussed. Your tank sits crosswise with the vent at the centre top of the tank end. Attached picture and drawing from the Grand Design Parts Look-up information. It is curious that driving up a grade with a 1/3 full tank would allow water to reach this vent connection.

Rob