Fresh water tank back filling thru pump

Sand, grit, or calcium (from some sources of hard-water) can cause the check-valve built into the exit-side of your water pump to stick slightly open. Then when hooked up to city/camp water it will back-flow thru the open check-valve and eventually overfill your fresh water tank.
First, turn the city water off at the faucet. Then turn your pump ON, then open a faucet in the lavatory or galley, etc. to allow the pump to push the debris out/thru the check valve. Operate the pump on/off/on/off a could times with the faucet open. Then with the pump flowing water close the faucet and turn OFF the pump.
Open the city/camp water and observe if that hasn't cleared the problem.

I've demonstrated this to several owners and it solved the problem for all but one of them. We had to disassemble the checkvalve for his pump and clean it out with vinegar, then reassembled it at it was fixed.

Hope this helps.

I thought it was OK, but seems to be doing it again. Very slow back flow, couple days to fill fresh water tank. The trailer is still under warranty, so when I get back from trip in October I'll have him fix it. Until then I'll do as you say and may be it fixes it self.

Thanks for the info.

Dont count to much on a new pump fixing your problem. I had that same problem even after a new pump was installed. I put in the check valve between the pump and the tank and no more problem.

Have you checked water pressure? s/b about 45 to 65 psi.

wnjj wrote:
...It uses check valves to push the water toward the outlet.

How does that work?

SaltiDawg wrote:

wnjj wrote:
...It uses check valves to push the water toward the outlet.

How does that work?

RV water pumps like the Shurflo models are positive displacement diaphragm pumps:

"A diaphragm pump (also known as a Membrane pump) is a positive displacement pump that uses a combination of the reciprocating action of a rubber, thermoplastic or teflon diaphragm and suitable valves on either side of the diaphragm (check valve, butterfly valves, flap valves, or any other form of shut-off valves) to pump a fluid."

Diaphragm pump

The graphic shows a 2-chamber pump, while the Shurflo and others have 3 or more chambers, but the concept for each chamber is the same.

Thanks for the info on the pump. Non RVer here.

Water pressure in is regulated at 44# and regulator has gauge showing out side pressure. If the new pump doesn't fix the problem I'll have to install a check valve as suggested.

Thanks for all the help and suggestions.

geneeich wrote:
Water pressure in is regulated at 44# and regulator has gauge showing out side pressure. If the new pump doesn't fix the problem I'll have to install a check valve as suggested.

Thanks for all the help and suggestions.

1. IF leaking thru the Pump, replacing the pump will FIX the problem. ALWAYS will.
2. ALL you have to do is remove the Intake fitting from the pump. IF leaking thru the pump, you will see water flowing out.
3. NO RV IS DESIGNED TO FILL THE FRESH TANK THRU THE WATER PUMP.
4. PSI has nothing to do with failure of the pump to stop backfilling.
5. Adding a check valve is NOT a hack if you intend to get the pump replaced. It is a good thing to do until you get the pump replaced. Doug