fresh water tank overflow syphoning tank dry

Lynnmor wrote:

FLY 4 FUN wrote:
I find this thread interesting. Our fifth wheel also has a pressure fill and valves to determine city water/system use/winterize etc. Last year I started noticing that when I fill the tank to capacity it would drain WAY longer than it used to when tap shut off at house. I would get to campsite and notice my "idiot light" was showing 2/3 tank. Not trusting it I ignored it. Now I show up at a campsite and find I can add 1 or 2 6 gallon jerry cans to what I believed was a full tank. In addition our family following us on road trips notes that we lose water on left turns and not just a little...looked like smoke coming off the tires. The last time I topped off the tank I noted water coming out overflow but when I got under the rig there is actually two red "overflow" tubes with one coming out hard/fast and the other trickling out. I wonder why there is two tubes? Is one an overflow and the other the breather tube? I would really like to understand them and may have to pull the coroplast to see whats changed as it used to just drain off a gallon or two as the tank was full and somewhat pressurized.

There are two VENT tubes to prevent pressurizing the fresh water tank. With a connection to city water, the incoming pressure can be quite high and the RV manufacturer does not supply a regulator. Two vents give greater capacity for out flowing air and also adds redundancy in case of one becoming restricted. NEVER add valves to these vents, if you ever forget to open them when filling, the tank will expand causing considerable damage. If you pump water from an unvented tank it will collapse and possibly dislodge from the supports. If your check valve in the water pump or the fill valve leaks, that too will fill the tank when connected to city water so the vents need to work always.

The fact that you didn't see the water spilling out in the past was just a matter of chance, straight roads cause less spilling.

The answer is to run the vents up and out of the coach well above the water level.

I added a gravity fill port of the hatch type. The elbows at the top are the required vents. The small holes in the door act as an insect screen and allows venting with the door closed and locked. The fill hose was attached to the tank by spin welding a hose barb to it.

I don't disagree that running the vents up and out is a good solution. However accessing the tanks to accomplish this is not a simple task.
Putting on valves is very easy. Tie a string to your faucet/finger or whatever it takes to remember to open the valves!

FLY 4 FUN wrote:
I find this thread interesting. Our fifth wheel also has a pressure fill and valves to determine city water/system use/winterize etc. Last year I started noticing that when I fill the tank to capacity it would drain WAY longer than it used to when tap shut off at house. I would get to campsite and notice my "idiot light" was showing 2/3 tank. Not trusting it I ignored it. Now I show up at a campsite and find I can add 1 or 2 6 gallon jerry cans to what I believed was a full tank. In addition our family following us on road trips notes that we lose water on left turns and not just a little...looked like smoke coming off the tires. The last time I topped off the tank I noted water coming out overflow but when I got under the rig there is actually two red "overflow" tubes with one coming out hard/fast and the other trickling out. I wonder why there is two tubes? Is one an overflow and the other the breather tube? I would really like to understand them and may have to pull the coroplast to see whats changed as it used to just drain off a gallon or two as the tank was full and somewhat pressurized.

There are two VENT tubes to prevent pressurizing the fresh water tank. With a connection to city water, the incoming pressure can be quite high and the RV manufacturer does not supply a regulator. Two vents give greater capacity for out flowing air and also adds redundancy in case of one becoming restricted. NEVER add valves to these vents, if you ever forget to open them when filling, the tank will expand causing considerable damage. If you pump water from an unvented tank it will collapse and possibly dislodge from the supports. If your check valve in the water pump or the fill valve leaks, that too will fill the tank when connected to city water so the vents need to work always.

The fact that you didn't see the water spilling out in the past was just a matter of chance, straight roads cause less spilling.

The answer is to run the vents up and out of the coach well above the water level.

I added a gravity fill port of the hatch type. The elbows at the top are the required vents. The small holes in the door act as an insect screen and allows venting with the door closed and locked. The fill hose was attached to the tank by spin welding a hose barb to it.

I find this thread interesting. Our fifth wheel also has a pressure fill and valves to determine city water/system use/winterize etc. Last year I started noticing that when I fill the tank to capacity it would drain WAY longer than it used to when tap shut off at house. I would get to campsite and notice my "idiot light" was showing 2/3 tank. Not trusting it I ignored it. Now I show up at a campsite and find I can add 1 or 2 6 gallon jerry cans to what I believed was a full tank. In addition our family following us on road trips notes that we lose water on left turns and not just a little...looked like smoke coming off the tires. The last time I topped off the tank I noted water coming out overflow but when I got under the rig there is actually two red "overflow" tubes with one coming out hard/fast and the other trickling out. I wonder why there is two tubes? Is one an overflow and the other the breather tube? I would really like to understand them and may have to pull the coroplast to see whats changed as it used to just drain off a gallon or two as the tank was full and somewhat pressurized.

MURPHY55347 wrote:
Yesterday they put a loop of tubing in the vent line that went above the tank then down. Gave me the coach back and said all would be good. No way it could siphon with a loop above the tank.

The way it appears now the tank is somehow pressurized.

Sounds like Winnebago doesn't understand a siphon. With a siphon it doesn't matter if they run a loop up high if the exit end is lower than the surface of the water. However if Winnebago had taken an extra step to cut or poke a small hole in the tubing at the top of the loop, then a siphon would be impossible. If worried about bugs glue a small piece of mosquito netting over the hole.

Based on your experience at the campground I would say the tank is somehow pressurizing. It would have been interesting to know if the flow would have stopped if you removed the fill cap to relieve any possible pressure.

We have a Vista LX 27N. Winnebago assures me there is no pipe on the vent that extends into the tank. If you look at their plumbing diagrams you will see that on the overfill pipe the use 1/2” tubing. Then to prevent siphoning they slip a larger diameter pipe over this and attach it with a screw. The theory is that the space created between these 2 different diameter pipes is supposed to let air in to prevent siphoning. Seems to be hit and miss wether it works or not.

MURPHY55347 wrote:
So we have a Winnebago Motorhome purchased last year. (snip)

I was surprised to hear it was a Winnebago. As they are one of the few manufacturers who actually fabricate their own tanks, specific to the model of the RV. I'm curious to hear what you have, since I purchased a new class A from them last year. I still think the only logical explanation of a siphon would be if the drain/overflow pipe somehow dropped into the tank, or was not installed correctly to begin with, and sits under the water level in the tank.

BTW, I have both a gravity and a pressure feed to fill the tank.No issues here (yet)

Lynnmor wrote:

MURPHY55347 wrote:

Today the change the angle of the 4” piece of tubing and raised it about an inch. Drove home and it appears to have worked. Will want to test more before any cheering. What I am still questioning is what part the gravity fill port might have to do with this.

The spill will continue till the water level drops to the vent port. Running the vent up hill by 1" will reduce spillage a bit. If the trailer leans to the side where the vent is connected, the spill will start, if leaning the opposite way there will be no spill.

A gravity fill system has a vent and fill pipe well above water level and that is why they work. A pressure fill system needs to have vents well above water level, but that would cost a few dollars that the manufacturers will not spend.

The manufacturer that I fought with changed back to gravity fill to fix the problem, but they still have no clue why water only runs downhill.

If you think that your problem is fixed, have a car follow close behind on a curvy mountain road so they can record the spillage. Yes, I did that.

I feel your pain. However I did not try to figure out what was causing the siphon. I really have no desire to remove the underbelly to get to the root of the problem. I am not located near the factory.
I simply put shark bite valves on the vent and overflow lines to stop the water loss. Yes I have to remember to open the lines when I get to the CG. Kind of a crude solution but it works for me.

MURPHY55347 wrote:

Today the change the angle of the 4” piece of tubing and raised it about an inch. Drove home and it appears to have worked. Will want to test more before any cheering. What I am still questioning is what part the gravity fill port might have to do with this.

The spill will continue till the water level drops to the vent port. Running the vent up hill by 1" will reduce spillage a bit. If the trailer leans to the side where the vent is connected, the spill will start, if leaning the opposite way there will be no spill.

A gravity fill system has a vent and fill pipe well above water level and that is why they work. A pressure fill system needs to have vents well above water level, but that would cost a few dollars that the manufacturers will not spend.

The manufacturer that I fought with changed back to gravity fill to fix the problem, but they still have no clue why water only runs downhill.

If you think that your problem is fixed, have a car follow close behind on a curvy mountain road so they can record the spillage. Yes, I did that.

So we have a Winnebago Motorhome purchased last year. Worked great all last year and most of this. My normal routine is to fill fresh water tank till it flows out the vent. It will run out for a bit then stop. Drive to campground and all is fine. Now get to campground back into my spot and water is running out the overflow. How can this be? There is no tube in the tank, just a 90 degree fitting off the top of the tank, a 4” piece of hose that goes horizontal to the tank and another 90 degree fitting with a hose going down under the coach. How can siphoning occur without a hose in the tank? Even if siphoning did start once water sloshed to the other side of the tank the vacuum would be broken. Was at the Winnebago factory yesterday and today and guess what. They don’t understand it either. Yesterday they put a loop of tubing in the vent line that went above the tank then down. Gave me the coach back and said all would be good. No way it could siphon with a loop above the tank. Nope! 70 miles down the road stopping at a wayside got out to look and sure enough it is running out the overflow. Turned around and went back. Today the change the angle of the 4” piece of tubing and raised it about an inch. Drove home and it appears to have worked. Will want to test more before any cheering. What I am still questioning is what part the gravity fill port might have to do with this. Never had one before this one. If that lets air in and siphoning starts it could contribute to the problem. If no gravity fill port, no air could get into the system and eventually there would be negative pressure in the tank stopping any siphoning. The way it appears now the tank is somehow pressurized.

Lantley wrote:
...this is the RV world where proper design and construction is low on the priority list.

Which is behind many if not most RV related problems.