Saturday, June 3, 2023


Why does fresh water tank fill up unexpectedly?

gary-736Dear Gary,
When I am hooked up to the campground water supply my fresh water tank fills up. How can I correct this problem? Thanks! —George B.

Dear George,
When the city water pressure feeds fresh water into the on-board fresh water tank it means one of two things: either a “quick fill” valve is left open (or is faulty), or the check valve in the outlet side of the water pump has failed. 

If your coach is equipped with such a quick fill valve (one that allows the fresh tank to be filled while connected to city pressure), it may have failed even if it is in the “off” position.

But it could also be that the backflow preventer (check valve), positioned at or near the outlet side of the water pump has failed. Though rare, this does happen from time to time. In most cases, this check valve is integral to the pump. (Note: A few RV pumps do not have this check valve as an internal component.)

It’s not necessary to replace the complete pump. Simply add an aftermarket, in-line check valve to the outlet fitting of the pump and leave the defective one in the pump if it cannot be removed. If it’s stuck in the open position it will still allow water flow and the new, in-line check valve will prohibit the city water pressure from back-filling the tank. Simple fix!

Read more from Gary Bunzer at the See Gary’s videos about RV repair and maintenance.




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Tony H.
2 years ago

Having the same issue. Was thinking of putting a shut off valve between the pump and the fresh water tank as I have easy access to the pump from inside cabinet and only use the pump while traveling from place to place. Would it hurt the pump if the pumps backflow failed and water went to the shut off valve I plan to install? My concern would be that the water pressure would be applied to the pump when the pump is off could damage the pump?

Shannon Daigle
2 years ago

Where are these quick valves located at? We just purchased a 2010 Gulfstream and when hooking to city water it is filling the freshwater tank.

August Degern
3 years ago

I have replaced the Diverter Valve and the water pump head. The tank still fills in the city water position. Also the water pump does not pump. Stumped, any help

4 years ago

Regarding the article, Why does fresh water tank fill up unexpectedly? This happened to us two years ago. We assumed it occurred because we left the water pump switch on by mistake and the (water pump’s) in-line check valve opened when we connected to city water. You do not mention this possible ’cause’ for a fresh water tank unexpectedly filling in your article. Could this have been a reason for it filling? We are now very careful and it has never happened again.

Richard Davidson
5 years ago

Have had this problem on my 97 Mountain Aire. The freshwater fill valve was leaking allowing water to enter the freshwater tank very slowly until it finally overflowed onto the ground. Solution was to take the valve apart (not as hard as it sounds) and replace the worn-out gasket inside. 20 minute fix. Most all older valves come apart and can be repaired.

August Degner
3 years ago

I replaced the valve. After I did this I tested the old valve and it was good.
The system had been working correctly since September of last year after I replaced the water pump that failed. I had not been used for about 3 months. When I went to use the pump it ran but did not pump. I open it up and found the valve inside was stuck. After I got this working smoothly I have the problem of the tank filling in the city position.

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