Plumbing riddle

[falconman]

I have a valve on the overflow tube. Any venting is handled via the fill tube.

[JCM]

This is how I solved the problem.

[gccch]

Excellent feedback on the valves!

Now, this problem has gotten a little more curious. After we figured out we could get full by stopping the fill before it overflows and thus avoiding the siphon, we moved a short distance across the park for one night. No problem. Maintained a full tank. We then drove a couple hundred miles to the next site and had water feed. No issue. After two nights we followed the same process to fill prior to overflow as we were planning a couple nights on the road. As we are driving around the winding roads of the Biltmore we noticed our water is spilling out the overflow. Not a continuous flow, but while turning and leaning (right turn, left lean). It is definitely flowing out the overflow tube! I checked. Twice.

So we lost about 2/3 of our fill along the route. Again, clearly not siphoning. Prior report was definitely siphoning at a stand still. So do I have two problems with this overflow tube?

I suppose the valve will fix the issue either way.

[CountryB]

I'm not familiar with your model coach, but it sounds from the description that you have two choices to fix this problem. Either add a Vent hole at the top of the overflow tube, or shorten the length of the vent tube inside the water tank.

[alank]

I don't understand why they would attach an overflow tube anywhere but at the top of the tank. In order for it to start a siphon of water out of the tank once the tube is full, it would have to be attached at some point below the top of the tank. Very poor design to say the least.

Assuming the tube is attached, or extends down in the tank, to some lower point, it would then have to go up to a level equal to the top of the tank before it is routed down through the floor. 530ktm reported putting a valve at that top position to break the vacuum and had good success with that.

A simple fill by fill fix would be to stick a small tube (hold finger on end of tube to retain air) up the overflow tube to allow air to get to the top of the overflow tube to break the siphon. If you attached the small tube to the bottom of the overflow and kept it capped, to retain air, you could just remove the cap to introduce air and break the siphon.

[alank]

Drilling a vent hole will allow water to come out that vent hole when water is up there. Just be sure a little water spill there won't be a problem.

Another thought is after a siphon is broken, and all is well, if you go on the road with a full tank, depending on where the overflow is attached to the tank, a jackrabbit start, a curve, or stepping on the brakes could put water pressure on the overflow tube in such a way to fill the tube and start the siphon again while driving. A valve on the bottom of the tube would be a solution there.

[hohenwald48]

Every overflow/vent tube I've ever seen originated in the the top of the tank. I can't understand how an overflow tube attached the the top of a tank could possibly siphon all (or even much) of the water out of the tank. A few gallons maybe but not much.

I could even understand how some water could "slosh" out the vent tube but only when moving the RV.

If your overflow/vent is connected anywhere other than the tank top then you would not be able to fill the tank. It would only fill to the level of the overflow/vent connection.

[wildmanbaker]

If you can look in the basement, and find the fresh water tank, it will be a milky white color, you might be able to locate the vent line, as the manual fill will be very close to the vent. If someone modified the vent line, or change the hook-up it may not be correct. As others have said, the vent should be in the very top of the tank, and not be able to siphon from the tank, unless it is sticking into the tank.

[BigBadDad]

Assuming you have external fill ports leave one or both open while you are running water into the tank...they will let water out and the siphoning should not be a problem. Mfr's often state this. Be sure to close the external ports as a last step after they no longer drip.

[harleyjt]

If your overflow/vent is connected anywhere other than the tank top then you would not be able to fill the tank. It would only fill to the level of the overflow/vent connection.

Actually, no. You could still fill the tank assuming the water inlet is at the same level or higher than the top of the tank. We used a bottom filling fuel tank on our full size vans for years. It took me a little while to understand it, but the level of the liquid in the tank will be at the same level as that of the liquid in the fill tube. So, if the fill tube comes into the bottom of the tank, the tube has to go up higher than the top of the tank. If the tube comes into the top of the tank, then it really doesn't matter.
jt

[bldrbob]

If you put a valve on the overflow make sure to open it when using water.The overflow is actually a vent to allow air in to replace the water going out.If the valve is closed the tank will collapse.The small line threaded to the high point sounds like a good idea. It would allow air in to break the siphon .

[gccch]

The siphoning is one thing, but please see post #17

http://www.irv2.com/forums/f258/plum...ml#post4050487