Water dripping out of fresh water tank overflow

When my tank was filled completely it expanded and then siphoned water until the tank contracted enough to break the siphon. I installed a simple siphon break to prevent this.

More recently the pump check valve is allowing a small back flush when on city water. I'm planning to install a second check valve in the line.

rgatijnet1 wrote:

dougrainer wrote:

rgatijnet1 wrote:
Not too unusual for the overflow to siphon out water from the tank. The tank on my Monaco did it so often that I finally just plugged up the overflow hose completely. That was six years ago and never had a problem. I have a vent right by the manual fill port so I saw no need to have an overflow hose that constantly siphoned off water from my tank.

Are you POSITIVE that the Vent next to your manual fill port is even connected to the Tank? Usually, when the OEM's connect a overflow that runs out the bottom, they do NOT run a 2nd overflow to the Fill port connection. Doug

When I fill my tank using the shore water and fill valve in the wet compartment I watch the manual fill port, located behind a door about midway up the body panel. Once the water starts to come out of the manual fill port, I stop filling the tank. The manual fill port has a vent located as part of the fill port. I did not say that the manual fill port was an overflow, I said it was a vent. If I do over fill the tank, excess water comes out of the manual fill port so to me it acts like a "full tank warning" and I shut off the hose filling my tank. Since it says that it is to manually fill my freshwater tank, and since water comes out of it when I am filling my fresh water tank with the shore water hose, I have to assume that the pipe is connected to the fresh water tank.
Here is a picture that shows the vent to the right of the gravity fill valve. I have to assume that it is a vent to the water tank. In six years with this coach I have never had a problem with the tank failing to pump water because of a vacuum condition.

I understand what you stated. MY point was, That vent next to the fill hose, is usually not connected, IF there is a vent line from the top of the fresh tank that is coming out the bottom of the RV. The pic shows the Vent, but as I stated, it is probably NOT connected. The REASON most RV makers vent the tank direct thru the floor to the outside is to prevent blowing up or bulging the Fresh Tank if the RV has City Water Tank filling option. IF the tank is not vented all the time, then you can either crack the tank or bulge it enough to destroy and bend frame supports around the tank. With your Pic of the standard Gravity Water Fill, they all come with the small vent connection. Some RV makers do run the vent hose up to that Gravity Water fill, and when of City Water Fresh tank fill, if you are not monitoring it, with the door CLOSED AND the 1.5 inch cap installed, the water pressure will build up and either pop the 1/2 inch line off the back side of the door and that water will flood your RV. Doug

dougrainer wrote:

rgatijnet1 wrote:
Not too unusual for the overflow to siphon out water from the tank. The tank on my Monaco did it so often that I finally just plugged up the overflow hose completely. That was six years ago and never had a problem. I have a vent right by the manual fill port so I saw no need to have an overflow hose that constantly siphoned off water from my tank.

Are you POSITIVE that the Vent next to your manual fill port is even connected to the Tank? Usually, when the OEM's connect a overflow that runs out the bottom, they do NOT run a 2nd overflow to the Fill port connection. Doug

When I fill my tank using the shore water and fill valve in the wet compartment I watch the manual fill port, located behind a door about midway up the body panel. Once the water starts to come out of the manual fill port, I stop filling the tank. The manual fill port has a vent located as part of the fill port. I did not say that the manual fill port was an overflow, I said it was a vent. If I do over fill the tank, excess water comes out of the manual fill port so to me it acts like a "full tank warning" and I shut off the hose filling my tank. Since it says that it is to manually fill my freshwater tank, and since water comes out of it when I am filling my fresh water tank with the shore water hose, I have to assume that the pipe is connected to the fresh water tank.
Here is a picture that shows the vent to the right of the gravity fill valve. I have to assume that it is a vent to the water tank. In six years with this coach I have never had a problem with the tank failing to pump water because of a vacuum condition.

It would be very unusual for the overflow to siphon out water from a tank that's less than half full, same with thermal expansion. City water fill valve or pump check valve leaking and letting the tank fill up is more reasonable, though the gauge would presumably indicate a more full tank and attempting to fill would of course immediately result in a stream out the overflow if it is indeed full.

If the gauge reads in percentages, rather than just having thee or four lights, then it presumably is a capacitive gauge and should be pretty reliable and accurate unless the sensor got detached from the outside of the tank. Even the sensor probe ones tend to be reasonably reliable in the fresh water tank since there isn't (supposed to be) muck to foul the sensors and cause false readings.

Is it possible the drip is coming from something other than the overflow? Might it be condensation on the tank, or a slight drip from a low point drain valve or tank drain valve that isn't fully closed or is leaking, or something of that sort?

rgatijnet1 wrote:
Not too unusual for the overflow to siphon out water from the tank. The tank on my Monaco did it so often that I finally just plugged up the overflow hose completely. That was six years ago and never had a problem. I have a vent right by the manual fill port so I saw no need to have an overflow hose that constantly siphoned off water from my tank.

Are you POSITIVE that the Vent next to your manual fill port is even connected to the Tank? Usually, when the OEM's connect a overflow that runs out the bottom, they do NOT run a 2nd overflow to the Fill port connection. Doug

If you are hooked up to city water you may have a check valve leaking the pressurized water into your tank.

Or it could just be the atmospheric warming of the tank/water causing expansion and pushing a small amount out the overflow.

interesting, never seen that one before after 14 years with my RV

Not too unusual for the overflow to siphon out water from the tank. The tank on my Monaco did it so often that I finally just plugged up the overflow hose completely. That was six years ago and never had a problem. I have a vent right by the manual fill port so I saw no need to have an overflow hose that constantly siphoned off water from my tank.