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Old 05-08-2022, 10:49 AM   #1
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Can I add an anti-siphon to my overflows?

Finally got around to pulling the corroplast down from under my fresh tank. Turns out, that was easy. They used a nice wooden strip across the main frame members and used normal screws to attach the corroplast. Nice.

I have always had issues with my overflows on my fresh tank when I filled up. They would siphon a lot of water out of the fresh tank. Now that I can see the install, I can see how a lot of slosh going down the road combined with siphoning would do that.

My question: Would there be any benefit to trying to install anti-siphons where the overflow exits the tank at the side? I may be able to get that elbow off, but maybe not. It was installed before the fresh tank was installed and there isn't enough room to turn the elbow because it hits the flooring above. I might be able to find a way to remove it if the recommendation is that an anti-siphon would help.

There are two overflow outlets, one on each side.

Whadya think?

The overflow is the one on the right. The one on the left is the fill.
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Old 05-08-2022, 11:26 AM   #2
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Holding tank "overflow" is typically the tank vent tube and you say you have 2. I see a vent (overflow), a fill and either a drain or water pump tube. If a drain could the other side tube be the water pump feed? Just thoughts. Siphoning has been an ongoing pia for a long time.
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Old 05-08-2022, 12:23 PM   #3
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Holding tank "overflow" is typically the tank vent tube and you say you have 2. I see a vent (overflow), a fill and either a drain or water pump tube. If a drain could the other side tube be the water pump feed? Just thoughts. Siphoning has been an ongoing pia for a long time.
The second over flow is on the other end of the tank.
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Old 05-08-2022, 04:52 PM   #4
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Well, if you have 2 overflows just cap off the one that is giving you issues. Only one vent is needed, and the other overflow can act as that.
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Old 05-08-2022, 11:41 PM   #5
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A anti siphon/check valve can not be used on a tank vent. When filling the tank the air in the tank must be able to exit. When drawing water out of the tank air must be able to flow in or the tank will collapse due to the negative pressure inside.
The vent hose should rise a few inches above the top of the tank before turning and going down below the tank, this will prevent siphoning. This may not be possible in your situation.
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Old 05-09-2022, 04:47 AM   #6
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A anti siphon/check valve can not be used on a tank vent. When filling the tank the air in the tank must be able to exit. When drawing water out of the tank air must be able to flow in or the tank will collapse due to the negative pressure inside.
The vent hose should rise a few inches above the top of the tank before turning and going down below the tank, this will prevent siphoning. This may not be possible in your situation.
A check valve and anti-siphon valve are two different valves with different functions.
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Old 05-09-2022, 07:35 AM   #7
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The way the photo is posted, the top of the tank is at the bottom of the photo and the elbow at the lower right with the blue pex is your vent/overflow, correct?
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Old 05-09-2022, 08:56 AM   #8
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The nearly 50 years of owning/remodeling/maintaining this house hasn't given me a bit of experience with what I've chased in my trailer. My trailer is gravity feed, so we're on different pages there also.

I don't see a siphon issue (unless there is a tube inside the fitting going lower in the tank). Water can't drain below the level of the fitting. That said, I can see that driving and sloshing can slop water out the drains.

My manual solution would be to put a valve on each of the overflows. Fill the tank. Close the valves. Drive all over and slosh the tank without losing a drop. Park, open the valve and you're in business.
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Old 05-09-2022, 10:07 AM   #9
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Overflow

I would never put a valve on an overflow. You are asking for trouble. Now a siphon preventer is perfectly fine. I bench tested mine with the tank out. Works fine, allows me to keep the tank full after driving all day, but allows for any pressure out. (I had to remove the tank because Cardinal didnít properly support the tank, so I made a support and cured the low tank capacity problem.)
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Old 05-09-2022, 10:20 AM   #10
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just add a longer overflow line and loop it above the tank .
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Old 05-09-2022, 11:17 AM   #11
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just add a longer overflow line and loop it above the tank .
To this point, are the vent hoses higher than the fill hose and hardware end to end?
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Old 05-09-2022, 11:34 AM   #12
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I would never put a valve on an overflow. You are asking for trouble. Now a siphon preventer is perfectly fine. I bench tested mine with the tank out. Works fine, allows me to keep the tank full after driving all day, but allows for any pressure out. (I had to remove the tank because Cardinal didnít properly support the tank, so I made a support and cured the low tank capacity problem.)
Most folks I know (including myself) have valves on their fresh tank overflow due to siphoning/slosh whatever you want to call it.
On a gravity fill system it is not a problem if you forget to open the valve. Gravity fill caps are not air tight, so when using water from the tank, it will not pull a suction. When filling with the valve closed, it will remind you to open it when it pukes water back at you when about half full if you are filling too fast.
On the pressure/pump fill systems you do have to be aware of what you are doing to ensure the valve is open when using/filling tank, so a siphon preventer might be safer.
Always have to be smarter than what you are playing with.
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Old 05-09-2022, 11:44 AM   #13
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In the picture, the two hoses coming out of the top is the top of the fresh water tank. The one on the left is city water fill. The one on the right is the overflow. There is the same thing on the other end of the tank.

The hose from the overflow elbow just goes straight down through the corroplast and ends just below the corroplast. I made a mod where I added a hose to the end of the overflow hose and brought that back up along the outside of the frame and attached it at floor height. Did that on both sides. I think that may have alleviated any siphoning. I did notice we didn't lose nearly as much water with the added hose going back up to the top of the tank level. Maybe I am chasing ghosts.
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Old 05-09-2022, 12:21 PM   #14
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I'm no hydrodynamics professor, but wouldn't a vent above it all solve your problems?
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Old 05-09-2022, 03:08 PM   #15
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All that's necessary to prevent siphoning out water is a tiny vent. I'd try rotating the overflow fitting so it points a bit above horizontal then install an extension to the drain tube that you attach to the floor above the tank then to the original drain tube. Once you know where the attach point is, drill a small (1/16 or 3/32) hole in the top of the tube under the clamp at the attach point. This tiny vent will break the vacuum that forms when a slosh of water fills the overflow tube with liquid. Putting the hole under the clamp will help prevent dust from entering the tube.

I did this with the overflow on my 2011 Georgetown and completely eliminated siphoning while traveling.


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Old 05-09-2022, 03:21 PM   #16
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All that's necessary to prevent siphoning out water is a tiny vent. I'd try rotating the overflow fitting so it points a bit above horizontal then install an extension to the drain tube that you attach to the floor above the tank then to the original drain tube. Once you know where the attach point is, drill a small (1/16 or 3/32) hole in the top of the tube under the clamp at the attach point. This tiny vent will break the vacuum that forms when a slosh of water fills the overflow tube with liquid. Putting the hole under the clamp will help prevent dust from entering the tube.

I did this with the overflow on my 2011 Georgetown and completely eliminated siphoning while traveling.


Phil
That sounds like a great idea. Will give that a go.
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Old 05-10-2022, 07:39 AM   #17
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The nearly 50 years of owning/remodeling/maintaining this house hasn't given me a bit of experience with what I've chased in my trailer. My trailer is gravity feed, so we're on different pages there also.

I don't see a siphon issue (unless there is a tube inside the fitting going lower in the tank). Water can't drain below the level of the fitting. That said, I can see that driving and sloshing can slop water out the drains.

My manual solution would be to put a valve on each of the overflows. Fill the tank. Close the valves. Drive all over and slosh the tank without losing a drop. Park, open the valve and you're in business.
That would've been my recommendation.
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Old 05-10-2022, 10:12 AM   #18
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A Valve on the overflow is kind of like a crank-up TV antenna. Will work great until you forget to open it when filling. Just like lowering the antenna before moving.
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