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Old 07-22-2014, 11:50 PM   #1
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2005 24SS: Fresh water siphons out after filling - Fixed!

Originally, when I'd fill the fresh water tank on my 2005 24SS, about 4 to 5 gallons would run out of the overflow tube after I shut off the hose. That seemed like a big waste to me

I eventually discovered the sliding access panel to the vent tube. It's at floor level in the wall on the right as you enter the bathroom. The vent tube comes of the top of the fresh water tank, arcs up about 18", then arcs down through the floor to the outside. This produces a siphoning effect when the tank overflows out the vent tube, draining out those 4 to 5 gallons.

I eliminated the siphon by installing a small vent tube in the big vent tube. Yes, that's weird, but it works: now, only a quart or two siphons out. It's simple: I poked a small hole in the large vent tube, pushed in a plastic 1/4" straight barb fitting, then put a 20" piece of 1/4" tubing on the free end of the barb. The 1/4" tubing was run upwards and secured in place.

Now, after filling, there is so much water left in the tank, the original vent line has about 10" of water in it.
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Old 07-23-2014, 08:38 AM   #2
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Great "out of the box" solution to a problem that many have with the Jayco FW tank vent tube siphon issue...please post pics if/when able so we can see your fine mod!!
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Old 07-23-2014, 08:40 AM   #3
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Novel solution. I would keep an eye out for water sloshing out the top of the new vent mod while on the road.
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Old 07-23-2014, 10:56 AM   #4
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I have seen several threads describing fresh water tank siphoning water out while traveling. There has to be more to the problem than is being understood by these people.

Let's look at some facts. First, for a siphon to operate one end of the tube must be submersed in the supply. The other end must be lower than the supply level. The tube must be full (primed) to start the flow. When the supply end is no longer submersed in the supply the siphon action (flow) will stop and the siphon tube will empty. The siphon action will not restart by simply re-submersing it into the supply. It must be re-primed.

Now, with those facts in mind understand that the fresh water tank vent tubes are connected to a port in the top of the tank. There is no tube extending down from the top inside the tank. Therefor it is not possible to siphon water out of the tank through the vent tubes.

It would be possible to loose some water through the vent tubes due to sloshing during travel but that loss should be minimal. Nothing to match the descriptions of loosing 2/3 of their water while traveling.

Either the descriptions are being exaggerated or there is something else at play that has not been discovered. But it is NOT vent tube siphoning.
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Old 07-23-2014, 11:50 AM   #5
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I agree with tnchuck100. The water can only siphon out of the tank until it sucks air and then it must be primed and the suction point below water level to restart.

Here is my theory: The freshwater tanks are of thinwall construction and rectangular in shape so there is a lot of flex in the walls. When you fill the tank, you create a very slight positive pressure (along with the weight of the water itself) in the tank that causes the walls to 'balloon' out slightly. The tank continues to fill up and ultimately begins to flow out the vent tube filling it with water. You stop adding water but the tank has ballooned slightly out of shape. As the tank walls contract, the siphon has been established and continues to suck water out of the tank until ultimately the siphon is broken when it belches air back up through the vent tube.

On my 23B the factory vented the tank just as the OP stated, there is a small arc in the tube from the top of the tank up and then back down through a hole in the floor. There is a barb on my gravity fill port with a brass screen on the outside where the tank vent is supposed to be attached. The reason why it wasn't utilized at the factory is simple: cost down. It requires about 3 feet of 1/2" braided nylon reinforced tubing to reach the filler neck vent point and only about 18" to arc up and back down to the floor. I guarantee you a bean counter at the factory decided to make the change to save $$.

Tonight I am going to re-route the tubing up to the gravity fill assembly vent port on my trailer not because I'm losing water but because I don't like to get on my knees to look under the trailer to see if the water is coming out of the vent.

The other problem with the factory not using the gravity fill assembly vent port is if you miss the big hole you can shoot water in the vent hole which will get the floor inside the trailer wet.
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Old 07-23-2014, 03:28 PM   #6
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Quote:
Originally Posted by tnchuck100 View Post
Now, with those facts in mind understand that the fresh water tank vent tubes are connected to a port in the top of the tank. There is no tube extending down from the top inside the tank. Therefor it is not possible to siphon water out of the tank through the vent tubes.
I lose 4 to 5 gallons out the vent tube after I've shut of the hose, so it is possible to siphon water out of the tank through the vent tube. Here's my explanation:

Remember, the tank and vent tube are completely full of water when I see water coming out of the vent tube. The siphon effect in the vent tube pulls water out of the tank, the suction pulls the tank walls inward, so water is still present at the vent outlet of the tank. Eventually, the suction is no longer able to pull the tank walls inward, and the tank expands, pulling in some air. End of siphon effect.
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Old 07-23-2014, 03:40 PM   #7
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Originally Posted by rtbreeves View Post
Great "out of the box" solution to a problem that many have with the Jayco FW tank vent tube siphon issue...please post pics if/when able so we can see your fine mod!!
The first picture shows the "siphon breaker tube", placed outside the wall so you can see how it's situated when it's inside the wall.

The second picture is shows the barb attachment for the siphon breaker tube, which is stiff 1/4" ID polyethylene tubing.

I haven't had any water come out the breaker tube when filling or when driving the three times I checked. It's just so much easier for water to go out the big vent tube, I don't think there is any chance of getting water out the breaker tube.
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