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Fresh Water Tank Filling From City Water With Valve Closed

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My friend owns an '06 Mountain Aire (gas). His problem is: when plugged to city water and the valve to city water, the fresh water tank will fill up. It may take 7-10 days for the tank to fill up and overflow.

I suspect the valve (the one we turn to city water or tank fill) to be defective. A tech pretends it would be the valve where the city water hose is connected to the MH.

Any idea?

Thanks everyone for your help and Happy New Year motor homing.

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Possibly the pump is allowing city water to backflow through it. The pump may need to be rebuilt or dirt (sand) could be causing the leakage. Suggest that you remove pump, disassemble and clean parts. Put the pump back together and test. Rebuild kits are available for most pumps. Check online for your particular manufacturer and model number of your pump.

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Yes, there is a check valve built into the pump, and as stated above it may be weak, defective, or just some debris in the valve, which can cause the situation you described to happen.

I always install a commercial grade check valve after the pump because they are much more reliable than the one built into the pump.

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I've had several water pumps that did this. I had one factory rebuilt under warranty, it was leaking back into the freshwater tank and overflowing that tank again. I went back to the original pump that was installed in our motor home and no more problems. The leakers were Sureflo, big name, big advertising. I gave them a chance to fix it bit it still allowed backflow. So I'll never buy one again.

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The pump is not used in the City Water position. What we are talking about is that if the pump or check valve is not sealing correctly, water will backflow from the City Water connection through the pump into the fresh water tank. It is just a small maintenance problem that can occur.

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So if the water pump check value is leaking into the tank, if you were not connected to city water (pump now on) would you expect the pump to cycle periodically as the pressure side of the pump did leak back into the tank. Cycling more often the worse the leak was?

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I had that happen once and finally discovered the ball valve that controls the direction had twisted off and when the handle was turned the ball didn't move or only part way. I went to a plumbing supply and found the proper valve for less than $8. The same valve at a rv parts store was $37.

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Yes, failure to use a pressure reducer can lead to a lot more expensive and disastrous problems than a leaking valve. A busted water line can flood a coach! City water pressure varies widely, so always use a pressure reducer. There are high-flow pressure reducers available.

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My city water/fresh water fill valve is a Swan Quick Fill with a two positon lever. I too had my fresh water tank filling up when the Swan valve was in the city water position. I have a manual shut off valve between my water pump inlet and the fresh water tank. To eliminate the water pump check valve, I closed the manual valve and the fresh water tank was still filling. In fact, I could hear the water trickling into the tank with the Swan valve against the stop in the city water position. I took the handle off the valve and with a cresent wrench, moved the valve further than the handle would allow. Sure enough, the water stopped flowing.

It would seem the internal ball valve is just slightly off so that with the handle installed, it doesn't fully move to the city water position. It doesn't look like you can open the valve up to replace any internal parts and a new valve is $139! Guess I'll just keep using the cresent wrench for the time being.

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