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Quick RV Tech Tip: Why is my fresh water tank filling up when hooked to city water?

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Dear Dave, 
On the last two nights of a three-week trip, our city water connection started going into our fresh water tank and overflowed from there onto the ground. Can you tell us how this could happen? —James, 2018 Thor Chateau

Dear James,
It could be one of two issues, depending on your plumbing design. If you have a city water diverter valve that allows you to fill the fresh water tank with city water pressure while you are connected, the valve may not be closing all the way. This valve is a left/right valve that diverts the city water from the outside source, either to the plumbing for all the sinks, toilets, and showers, or when turned or diverted, directs water to the fresh water tank to fill it up before leaving. Without this valve, owners need to fill the fresh water tank by sticking a hose from a water source into the gravity feed on the side of the rig. These diverter valves can come loose and not close all the way when switched to city water, or there can be a calcium or lime obstruction that keeps them from closing.

The second issue could be a weak or clogged backflow prevention valve on the water pump. When connected to city water, the pressurized water is routed around the onboard water pump, but does have pressure on the back or outgoing side of the pump. There is a check valve there that is supposed to only allow water to flow out of the pump. If it is weak or, again, has calcium or lime keeping it from closing, pressurized water from your city source will flow back through the pump to the fresh water tank. Check out the article about that here.

##RVDT1976

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Jim Tibbles
1 month ago

Dear Dave & James
I have a Berkshire that did the same thing. I have a valve that I turn to city water, fill the fresh tank and tank water for rv. The valve was just slightly turned toward fill the tank when on city water. It was probably 1/8 ” toward tank fill. It was just enough that I had a trickle of water going into the fresh tank. It took about 1 or 2 days to fill the fresh tank and started out the overflow.
My point is to make sure the valve pointer is directly pointing toward what you want it to do.

Jim
1 month ago

Same thing happened to me and it was the selector switch. After replacing it I had no more problems.

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