Water Siphoning from Full Fresh Water Tank

[TedS]

[TedS]

Here is another idea.

[Country Campers]

The problem I see with the black tank valve is it only leaves air in then the water drains out of the system. The fresh tank needs to expel air when being filled and allow air to enter when the water is being removed all the while not letting any water to be lost while traveling. Not sure if that will be the answer but may be something to check out.

Brian

[Cate&Rob]

Vacuum break valves and/or raising the pipe to a much higher level can compromise the originally intended function of this vent/overflow. It is supposed to let air out while the tank is being filled and eventually let water out when the tank reaches full capacity. It then needs to let air in as water is drawn from the tank. It takes very little water pressure or vacuum to balloon or collapse these tanks. The siphon action is only going to happen until the water level in the tank drops below the vent hose port which is at the very top of the tank. What some encounter while travelling is not siphon action, it is slosh pumping. There are no baffles in the water tank so even slight motion will cause the water to slosh back and forth, "pumping" water out the vent line every time the wave of water hits that end of the tank.

Rob

[ncitro]

Yeah that was the problem, it needed to let air and water out (when filling), and air in (when using water). Someone suggested a irrigation vacuum breaker that would let air flow both ways but not water, but you still needed a valve to let water out while filling the tank. I wanted something completely automated that I could not screw up. My issue was compounded by how weak and thin the tank was. Any extension to the vent line would hold water and rather than break the suction the pump would just collapse the tank. I had this happen several times as I was trying different solutions, and actually had to blow into my tank with a air compressor to break the suction. I actually dropped my tank when I had the coroplast off and could not believe how flimsy the tank was. It actually had developed a bow between the braces. I added more bracing to try and limit that but when the tank was full it was almost like a balloon. What would be ideal would be to run the vent up to the roof like the holding tank vents. Then no siphoning, and no water can get stuck in the pipe. Thats also why when I ran my new vent to the back I made sure to pitch it uphill the whole way.

[Jlawles2]

Country Campers thats why it needs to be installed backwards. The breaker needs to think pressure is coming from the exit of the vent not the tank. I'm not 100% certain it would work.
Cate&Rob, been contemplating the vent thing while filing. I do not have my reflection yet (due off the line tomorrow 2/24) but I know me and will fill the tank as others waiting till i get a steady stream from the vent. This actually starts the siphon and its all out the vent from there. Since you like plumbing (pun very intended there) and probably have the supplies around to try this, what happens if the vent comes horizontally out of the tank, then where it makes the vertical down it either gets a Tee with the vent on the stem and one pointing up other pointing down? Or where it turns vertical the pipe undergoes a dramatic size increase of say 2x. I.E. 1/2" pipe goes to 1" or larger which will allow air to backdraft up the larger diameter breaking the siphon. To stop the siphon it only needs to get a continuous supply of air to the highest or closest point to the tank.

The thing that concerns me after thinking about this for a few days is collapsing of the tank. Plastic is easy to mold, but also stands a chance to get weaker each time its flexed. While it may not undergo the same fatigue failure like steel, it can fatigue fail. I would be worried that the tank in time will collapse completely.

[ncitro]

Country Campers thats why it needs to be installed backwards. The breaker needs to think pressure is coming from the exit of the vent not the tank. I'm not 100% certain it would work.
Cate&Rob, been contemplating the vent thing while filing. I do not have my reflection yet (due off the line tomorrow 2/24) but I know me and will fill the tank as others waiting till i get a steady stream from the vent. This actually starts the siphon and its all out the vent from there. Since you like plumbing (pun very intended there) and probably have the supplies around to try this, what happens if the vent comes horizontally out of the tank, then where it makes the vertical down it either gets a Tee with the vent on the stem and one pointing up other pointing down? Or where it turns vertical the pipe undergoes a dramatic size increase of say 2x. I.E. 1/2" pipe goes to 1" or larger which will allow air to backdraft up the larger diameter breaking the siphon. To stop the siphon it only needs to get a continuous supply of air to the highest or closest point to the tank.

The thing that concerns me after thinking about this for a few days is collapsing of the tank. Plastic is easy to mold, but also stands a chance to get weaker each time its flexed. While it may not undergo the same fatigue failure like steel, it can fatigue fail. I would be worried that the tank in time will collapse completely.

Yeah I can tell you the tank definitely flexes. See my comment above for the bowing I experience while filling and the collapsing from running the pump with the vent line (3/4") blocked with water.

[Cate&Rob]

Jlawles2


Part of the problem is that the overflow line must bring the water out of the underbelly. This is why there can't be a vent opening facing up.

The siphon action can only happen until the water level in the tank drops below the top of the vent pipe connection. This is within about 1/2" of the top of the tank. It is also possible that a tank could be pressure filled at a rate greater than the overflow line could accommodate. This would cause the tank to swell and once the fill was stopped, the tank should return to normal shape by pushing this extra water out the overflow. I think the Grand Design operation manual advises to stop filling as soon as water is seen at the overflow. Waiting for greater volume from the overflow could mean that you are ballooning the tank.

Rob

[TedS]

GD could redesign the vent plumbing to run from the tank uphill to the valve panel. That would always be open with no valves; would burp water where you would see it at the valve panel while filling; would not allow sloshing to dump water. That could be a DIY project.

[Country Campers]

Jlawles2

When filling the tank I do not think you want to fill until a steady flow is observed flowing from the over flow pipe. If waiting for a steady flow the tank may be expanded enough to cause damage to the tank or the fittings on the tank. I only fill my tank until I see any water coming from the over flow , drops or a trickle , then stop the filling process. Usually after stopping there remains a small trickle for a little bit then it stops. I do also have a valve on the overflow for travel.

Brian

[Cate&Rob]

GD could redesign the vent plumbing to run from the tank uphill to the valve panel. That would always be open with no valves; would burp water where you would see it at the valve panel while filling; would not allow sloshing to dump water. That could be a DIY project.

Given that the tank vent/overflow is located at the top of the frame rail on the inboard side, an uphill path to the valve panel would require a route through the floor and probably along/through an exterior wall. Certainly possible but complex. The outlet would have to be on the exterior of RV wall to avoid dumping water into the storage space where the valve panel is located.

Country Campers Brian's solution in post 25 is much easier to accomplish and recognizes that the issue is slosh pumping, not siphoning, for the relatively few owners who travel with a full fresh water tank. The concern is a "fail-safe" method of closing the vent line while travelling, but making sure that it is open whenever the RV is stationary. Hmmmm . . . there are electric solenoid valves that could be installed to accomplish this. The question is what electrical signal defines open vs closed?

Rob

[Jlawles2]

Cate&Rob what about a normally closed valve like a natural gas valve? Would need 1 switch and diode. Wire valve to switch in control panel fro filling and to pump control. One could be industrious enough to find the pressure switch on the pump and tap it there so the valve cycles with the pump.

Originally I was thinking normally open, but where do you tap the signal to close the valve?

[Cate&Rob]

Cate&Rob what about a normally closed valve like a natural gas valve? Would need 1 switch and diode. Wire valve to switch in control panel fro filling and to pump control. One could be industrious enough to find the pressure switch on the pump and tap it there so the valve cycles with the pump.

Originally I was thinking normally open, but where do you tap the signal to close the valve?

It would be easy to control this solenoid (either NO or NC) from a switch at the water panel. But, just like a manual valve, how do you assure that the switch is remembered every time you want to fill or draw from the water tank? Tapping into pump power would cover tank use, but what about filling? This valve could be wired to close whenever the trailer cord is plugged into the truck, but this would miss filling or using the water tank when hooked up to the truck.

As a reference, I use one of these valves in NC mode on my city water supply, wired to the electric water heater circuit. Thus, whenever we leave the trailer, I shut off the water heater at the inside panel and this closes the city water feed to the trailer. Much easier that remembering and taking the time to go around to the post and turn of/on the faucet each time. (Leaving the trailer with a connected and open city water supply is just asking for trouble)

Rob

[howson]

The siphon action can only happen until the water level in the tank drops below the top of the vent pipe connection. This is within about 1/2" of the top of the tank.

Isn't the reason the siphon action lasts longer than this implies is the tank collapses on itself as the water is siphoned with no air source to take the place of the water? Since the tank is collapsing the water stays with that 1/2" a lot longer than expected. The water seems to keep flowing until the tank reaches a point where the resistance of further collapsing equals the external air pressure surrounding the tank--right?

[Cate&Rob]

Isn't the reason the siphon action lasts longer than this implies is the tank collapses on itself as the water is siphoned with no air source to take the place of the water? Since the tank is collapsing the water stays with that 1/2" a lot longer than expected. The water seems to keep flowing until the tank reaches a point where the resistance of further collapsing equals the external air pressure surrounding the tank--right?

Hi Howard.

This could be possible, although the pressure differential driving the siphon action is only a few inches of water. Presumably, once this level of "vacuum" in the tank is reached, the vent line would "burp" allowing air back in, to stop the siphon action. Sort of like holding an open bottle of water upside down. This could definitely be affected by longer or higher vent line piping.

I think it is more likely that pressure filling to a strong stream of water from the overflow has ballooned the tank and the extended drain time is the tank pushing water out as it returns to normal shape and size.

It would be interesting to be able to open another vent to the tank to test your theory . . . I just don't know how to easily do that. It would be an interesting development project to try these things with a tank sitting on a workbench.

Rob