I know this is an issue with several trailers , and looking to start my spring to-do list... With my ( and many others) trailer , once the fresh water tank is filled , it will run out the overflow to prevent pressure build up... With ours , once that happens and I drive down the road , I lose a lot of water in transit.Up to now it has not been an issue , but with doing more dry camping I was thinking of what can be done... My thoughts were to add a water pressure reliefs valve on the overflow lines.... This would allow you to fill up , it would allow the pressure valve to open once full and close again.I have heard of people putting regular throw valves on , closing them and bursting something the next time they fill up.. These are on the tanks that do not gravity fill ports , just city water inlets with a swing valve to divert water to the tank...Joe

Are you losing more than a gallon of water? Otherwise, I'd just leave it alone!WoodGlue

Mine on the last TT I had did this and I thought of putting an on off valve inline of the pressure or over flow tube but I had second thoughts, I was afraid I would forget and blow the tank apart. so I just filled up before I left the last town and I also carried some extra 5 gallon jugs to make sure.

When I had the Triple E the fill tube was inside a carpeted compartment.. I finally figured it out by putting in a wine carboy tapered plug into the fill tube, and fed that into a half gallon jug. The overfill tube got the same treatment. Within a quarter mile of turns, the fill tube had spilled enough to almost fill the jug, and the overfill tube has spilled about 1/8 that amount. After that I devised a way to cap the fill tube tightly, and I just ran with the overfill tube into a jug with the hose able to breathe free air. I was then able to fill my tanks and run for the campground without worry.

Tank has created a siphon effect from filling.........due to lack of proper ventingCheck the vent line routing.Straight up as much as possible from tank, no loops, no kinks, no trapped water

What is going on is NOT a siphon, just a simple spill. The vent pipes are attached at the top of the tank and every time water sloshes that way it spills out the pipes. You can lose most of the water on a curvy road. The only real fix is to run the vents uphill and out of the coach well above the water level. The old gravity fill type had no problem because the vent was above water level. Read this months,(Jan 2014)Trailer Life for more information.

Fresh water loss while driving | Good Sam Community

Lantley
Nomad
Dec 17, 2013
Gjac wrote:
To all those that have installed ball valves to prevent siphoning I would caution against this. I Guy I traveled with for a while did this and when we pulled into a FHU CG he forgot to open the valve before he switch his city water valve and the pressure build up in the FW tank caused the tank to swell and it broke the supporting structure. I took a half a days work and a lot of FG repair to fix. I fixed my problem by raising the over flow hose above the tank to break the siphoning effect, It is an easier fix especially if you are older and forget to open the ball valve.

I won't debate what you to call it, But when my fresh tank is totally full if I don't close the valves a lot of the water will come out of the pipes beneath the trailer and fall to the ground.
When I arrive at my site I will have lost a lot of the water I was traveling with.
In order to control this situation I installed sharkbite ball valves on my vent and overflow lines that protrude through the bottom of the trailer.
Yes if I attempt to fill tank with valves close there will be damage.
That type of mishap is known as operator error.

There are an infinite number of scenarios, that operator error may result in major damage.
In the end it's a given that you must know what you are doing.
Lynnmor
Explorer
Dec 17, 2013
Gjac wrote:
To all those that have installed ball valves to prevent siphoning I would caution against this. I Guy I traveled with for a while did this and when we pulled into a FHU CG he forgot to open the valve before he switch his city water valve and the pressure build up in the FW tank caused the tank to swell and it broke the supporting structure. I took a half a days work and a lot of FG repair to fix. I fixed my problem by raising the over flow hose above the tank to break the siphoning effect, It is an easier fix especially if you are older and forget to open the ball valve.

Firsthand experience of what could happen. Thanks.

Once again:

It is not a siphon.
It is a spill.
They are not overflows.
They are vents.

Siphon

Note that a siphon upper tube must dipped into the water, not at the top of the tank. The RV tanks in question have "holes" at the top and water only flows out when the tank leans to that side or the water is sloshed in that direction.
Gjac
Explorer III
Dec 16, 2013
To all those that have installed ball valves to prevent siphoning I would caution against this. I Guy I traveled with for a while did this and when we pulled into a FHU CG he forgot to open the valve before he switch his city water valve and the pressure build up in the FW tank caused the tank to swell and it broke the supporting structure. I took a half a days work and a lot of FG repair to fix. I fixed my problem by raising the over flow hose above the tank to break the siphoning effect, It is an easier fix especially if you are older and forget to open the ball valve.
Lynnmor
Explorer
Dec 15, 2013
Ford truck guy, You can take any advice you want, but it is not safe to pressure fill your tank without vents that are two times larger than the supply line. The fresh water tank is not a pressure vessel and will fail at very low pressures. City water pressure can be anywhere from 30 to 100 PSI and that can and will cause severe damage. The frame, floor and tank may be damaged. It is irresponsible for anyone to give advice that may cause you severe problems. It is also irresponsible to sell or trade a unit that may cause others harm.

Again, it is not a siphon.
It is a spill.
You must vent the tank well above water level.
Lantley
Nomad
Dec 15, 2013
Lynnmor wrote:
ford truck guy wrote:
On ours , the tank is just about up to the floor boards , with the hose running out the top and straight down. There is no where for it to go up...

I replaced the tank with one that had an inlet on top right under the dinette. From there I ran the pipe outside.
If there is no way to go up where the pipes are, then attach long hoses to the current vents and run them to where you can go higher. Install low point drains as needed in the hoses.
Whatever you do, don't install valves to close off the vents, that is an accident waiting to happen.

While I don't disagree there is a potential for a mishap. It not as bad as you make it. The tank will be full at arrival. Water will not flow from the tank properly until you open the vent valve. The poor flow should alert the user to open the travel valves back up!
The real potential for a mishap is if you were to fill tank with both valves closed. However you will be alerted that there is a problem before you need to re fill the tank.
Ball valves are an simple solution,Sharkbite ball valves are an easy solution.
Lynnmor
Explorer
Dec 15, 2013
ford truck guy wrote:
On ours , the tank is just about up to the floor boards , with the hose running out the top and straight down. There is no where for it to go up...

I replaced the tank with one that had an inlet on top right under the dinette. From there I ran the pipe outside.
If there is no way to go up where the pipes are, then attach long hoses to the current vents and run them to where you can go higher. Install low point drains as needed in the hoses.
Whatever you do, don't install valves to close off the vents, that is an accident waiting to happen.
ford_truck_guy
Nomad
Dec 15, 2013
On ours , the tank is just about up to the floor boards , with the hose running out the top and straight down. There is no where for it to go up...
JaxDad
Explorer III
Dec 15, 2013
millerak49 wrote:
Ours would overflow and then create a siphon as well. I installed a ball valve. I keep it cracked just slightly all the time. some water will drip out while driving, but very little compared to without. there isn't enough room to get the vent line any higher above the tank. All roads are rough and curvy here in Alaska i think.

My overflow is a good 12" above the top of the tank.

I would need to have the RV tipped over on its side to create a siphon since the outlet needs to be below water level to create flow.
millerak49
Explorer
Dec 14, 2013
Ours would overflow and then create a siphon as well. I installed a ball valve. I keep it cracked just slightly all the time. some water will drip out while driving, but very little compared to without. there isn't enough room to get the vent line any higher above the tank. All roads are rough and curvy here in Alaska i think.
Lantley
Nomad
Dec 14, 2013
JaxDad wrote:
I'd be surprised that you get much out the overflow / vent pipe. They usually mount them a little ways above the tank to prevent just that from happening.

If though you are getting spillage, I would definitely suggest a simple floating ball check valve over a standard ball valve. First because it's automated so there's nothing to forget to do. More importantly though because to eliminates the possibility of damage to the plumbing in the RV.

A floating ball check valve is a little device that looks a lot like an inline fuel filter. Inside the body of it is a little hollow plastic ball that gravity drops down into a recess to mostly close off the opening, enough air can pass to allow for normal operation though. When however water comes up the line the ball floats, rises into an upper recess and again mostly blocks the opening. Enough water can come out to tell you the tank is full, not enough to cause an issue.

I wonder though if the real issue isn't that your RV, like mine, has twin water tanks. In my case one under each dinette bench. There's a crossover line about 3/8" between the two to balance them out. However that little line underno pressure save the water weight itself is extremely slow to equalize the tanks. If I fill the water with a hose and stop when water appears I have one tank full and one tank 1/4 full. If I leave it sit for a while and recheck, I have a reading of 1/2 a tank of water because it's now divided between the two.

I can't speak for everyone but my tank will overflow and create a siphon want the water starts to flow. Ball valves are a simple solution that solves the problem. Yes you need to remember to open them upon arrival

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