A question about fresh water tank overflows

[Johnny]

This should be a quick, easy question, but since I'm new to the TT world, I need to ask this.

My 26BH has the two 42 gallon tanks. I know, of course, to fill them up until water starts to run out the overflows. Also, I've already installed shut off valves on the overflow hoses for travel.

My question is, how much water should I expect to run out the overflows if I shut the hose off immediately? I only ask because I shut the hose off within 30 seconds of water coming out of the overflows and I would guess that around 3 - 4 gallons drained out.

Does this sound about right?

[Midnightmoon]

I noticed that it runs out for quite a while on our new Jay Feather also.

[Jagiven]

it varies, because you can overfill and pressurize the tank a little bit.

If I am paying attention, I can hear when the tank is just about full, as there is a couple gurgles before water will start coming out of the overflow pipe. It also depends on the water source. At home we have terrible water pressure (36psi), and the flow is fairly slow. Again if I am paying attention very little water will come out after I turn off the water (maybe a half gallon). At some of the state parks with high water pressure and high flow, I might get a couple gallons of water come out of the over flow, and I will get a burp of water out of the gravity fill when I turn off the water supply.

[dgodecke]

I have a valve on my overflow also but I make sure the overflow has stopped before I close it and head down the road. You need to have all the pressure out of the tank(s) before you block the overflow. Remember you do also have a small vent line. Never block that line as it compensates by letting air into the tank when you use water but also when you are traveling in mountains the the barometric pressure changes with altitude changes.

[RVhiker]

Originally Posted by Johnny

..... My question is, how much water should I expect to run out the overflows if I shut the hose off immediately? I only ask because I shut the hose off within 30 seconds of water coming out of the overflows and I would guess that around 3 - 4 gallons drained out......

I have two 42 gallon tanks as well. My experience has been that if I shut off the hose as soon as the two overflow lines are flowing water that many, many gallons of water will be drained out. I believe that the lines are siphoning. A loss of only 4 - 5 gallons would be a great improvement for my trailer. Maybe my overflow lines are set lower on the tank than yours?

[dgodecke]

More likely when you fill the tanks you create a small amount of pressure. This occurs when the tank gets full. the water in the overflow is not able to get out as fast as the water under pressure you are using to fill the tank. the result is the overflow continues to run until that pressure is diminished. When the outflow stops running you will have full tanks and no residual pressure in the tank.

[Johnny]

Originally Posted by dgodecke

I have a valve on my overflow also but I make sure the overflow has stopped before I close it and head down the road. You need to have all the pressure out of the tank(s) before you block the overflow. Remember you do also have a small vent line. Never block that line as it compensates by letting air into the tank when you use water but also when you are traveling in mountains the the barometric pressure changes with altitude changes.

Can you tell me where this vent line may be located? That could be useful information.

[Johnny]

Originally Posted by RVhiker

I have two 42 gallon tanks as well. My experience has been that if I shut off the hose as soon as the two overflow lines are flowing water that many, many gallons of water will be drained out. I believe that the lines are siphoning. A loss of only 4 - 5 gallons would be a great improvement for my trailer. Maybe my overflow lines are set lower on the tank than yours?

I had read on here about the overflow siphon effect, so when they didn't stop draining after a couple of minutes or so I closed the shut-offs on the overflows, went into the camper, turned the pump on and then turned on a faucet for about 15 seconds.

When I went back out and re-opened the overflow valves it only drained about 30 more seconds and then stopped.

I'm not sure if that made a difference or not. I guess I was thinking that maybe I could introduce a back pressure and stop the siphon effect. Not knowing for sure where the overflow hoses connect to the tank, I can't be sure I wasn't just wasting time.

[PonyExpress]

ive been wondering myself if it starts a siphon when over filling that actually drains down below full before stopping.

What exactly is the difference between the vent line and the overflow line?

[Tardis-1]

I got tired of the overflow mess. I have a much smaller tank, but I installed an LED light and WiFi backup camera in the water tank compartment, powered by the water pump 12V with an added power switch.

I watch on my cell phone as the water fills and stop near the top. Never an overflow. Also, I can see the exact water level at any time by just firing up the camera and cell phone.

The camera is aimed to just see the bottom of the tank and the top where the fill hose exits.

One picture shows a bit of water in the tank (brighter white) and how it appears on the phone.

I added 2-gallon tape marks later to know the exact amount left.

[dgodecke]

Originally Posted by Johnny

Can you tell me where this vent line may be located? That could be useful information.

On my 5th wheel it is right next to the overflow pipe and is a small white hose, 3/8 ID, 1/2" OD. The dealer extended that line and put a loop in it to stop any water loss. He claimed that was the problem and fix for water loss...It was not.

it is designed to let air in when you are using water and to balance pressure when you go up or down in altitude.

[Johnny]

Originally Posted by dgodecke

On my 5th wheel it is right next to the overflow pipe and is a small white hose, 3/8 ID, 1/2" OD. The dealer extended that line and put a loop in it to stop any water loss. He claimed that was the problem and fix for water loss...It was not.

it is designed to let air in when you are using water and to balance pressure when you go up or down in altitude.

Hmmm.. I guess I don't understand that. It seems to me that the overflow hose or pipe would serve that purpose just fine.

[Subaru297]

I am curious where the overflow line drains from? The bottom of the tank or the top?

I haven't experienced this siphoning but have read about it happening a fair bit. Siphoning 1/3 or 1/2 of your tank would be impossible if the intake for the overflow was at the top of the tank. Or is it located on the side somewhere?

Cheers

[Johnny]

Originally Posted by Subaru297

I am curious where the overflow line drains from? The bottom of the tank or the top?

I haven't experienced this siphoning but have read about it happening a fair bit. Siphoning 1/3 or 1/2 of your tank would be impossible if the intake for the overflow was at the top of the tank. Or is it located on the side somewhere?

Cheers

I have to agree with you. It would only make sense for an overflow to be at the top of a tank, and in that case there shouldn't be any siphoning occurring.

If an overflow were located on the top of the left side though, then I can see centrifugal force pushing out quite a bit of water during right turns. If there were a lot of right turns you could lose a lot of water over the course of a trip.

I guess it would make sense for it to be located in the middle of the top of the tank where turning, braking and acceleration would have the least impact.

[RPreeb]

Originally Posted by dgodecke

More likely when you fill the tanks you create a small amount of pressure. This occurs when the tank gets full. the water in the overflow is not able to get out as fast as the water under pressure you are using to fill the tank. the result is the overflow continues to run until that pressure is diminished. When the outflow stops running you will have full tanks and no residual pressure in the tank.

This is a good theory, but it's wrong, at least in my case it is. Mine will nearly empty the tank if I let it go. If I don't shut off the ball valve I installed on the overflow, it just keeps draining until there is almost nothing left in the tank.

[dgodecke]

Originally Posted by RPreeb

This is a good theory, but it's wrong, at least in my case it is. Mine will nearly empty the tank if I let it go. If I don't shut off the ball valve I installed on the overflow, it just keeps draining until there is almost nothing left in the tank.

WOW! that sounds like they left hose inside your tank when they installed the overflow. You really need to take that back to the dealer and have it fixed. What you have is the exact definition of a syphen.