Problem! Freshwater Tank Unexpectedly Filling When on City Water

SHURflo RV Water Pump

We were at camp one day when we noticed water was coming out of the overflow vent for our freshwater tank. It didn’t make any sense as we were hooked up to city water at the campsite and hadn’t used the freshwater tank in a couple of weeks. So why was my freshwater tank suddenly overflowing?

What’s Happening?

Turns out, this is a fairly common occurrence. There is a one-way check-valve inside of the water pump and this often gets dislodged and stuck open. When this happens, water from the city water connection can flow backwards through the pump and into the freshwater tank.

Possible Causes

The 2 biggest causes of this is:

  1. Having the pump on and connected to city water at the same time
  2. Being on city water with extremely high water pressure without a water pressure regulator
  3. Or, good ole fashion pump failure (look for replacement pumps)

Possible Fixes

Many times, the simplest fix is to cycle between the water pump and city water a couple of times:

  1. Turn off campsite water
  2. Turn on water pump
  3. Run water at a faucet for 10-15 seconds
  4. Turn off water pump
  5. Turn on campsite water
  6. Run water at a faucet for 10-15 seconds
  7. Repeat, 2-3 times.

Often times, this is enough to reset the check-valve inside of the water pump.

If it happens repeatedly, there are a few solutions to fixing it:

  1. Check for a “rebuild kit” for the water pump and find out if the internal check valve is a user-serviceable part (some art, some are not).
  2. Add your own 1-way check valve to the water line on the inlet side of the pump.
  3. Go whole hog and replace the pump itself. Check Amazon or RV Water Filter Store for options.
    If you don’t know what you need, RV Water Filter Store’s customer service is fantastic. Many moons ago, the owner of the site called me on a Sunday evening to help me decide what I needed.


So there you go… if you’re at camp you notice water coming out of the freshwater tank’s vent and are hooked to city water, it very likely is that water is back flowing through the water pump. As you can see, the fixes are pretty easy.


  1. Good tips, but there is an additional (common) cause that you didn’t touch upon. If the RV is equipped with a valve to fill the fresh water tank from the city water hookup, often times that valve will leak, allowing city water into the tank.

    • I replaced the diaphragm in my sure flow pump and it has check valve functionality when it’s in good shape that was $15 I also spent 17 on a check valve which would need to be installed on the discharge side of the pump because that’s what’s happening the discharge side is common to your city water so it’s flowing backwards through the impeller when it’s worn out. Symptoms are the pump acts like it’s losing pressure through about or a water fitting even though the faucets are off it’s just worn out folks. Secondly the diaphragm pumps are almost bulletproof but after 27 years even they wear out you can’t tell by looking at them but for 15 bucks I got the entire ahead internal workings with the valves and the diaphragms and it was the fix. I can even now hook up to city water and the pump will not cycle even with the faucets running because it doesn’t drop long enough and it’s not leaking through. My 2cents. Sure flow as featured in the picture above does not contain a check valve but it works like one.

  2. You say in the three quick fixes, to install a one way check valve on the input side of the pump. Shouldn’t it be the OUTLET side? Because that’s where the water is is being pushed into from the city water inlet. The inlet side should be connected directly to the fresh water tank. Anyway, I like one poster’s idea of installing a shut-off valve directly in the output side of the pump!

  3. We saw water running out of the Fresh Water Fill connection. We don’t use fresh water or city water. We drained the fresh water tank. Unfortunately water did get inside the camper ! How is that happening?

    • Water shouldn’t get inside of the camper when this happens. You need to track down where it came from. Start with getting eyes on the freshwater tank and follow the overflow tube and the tank vent.

      Typically when this happens, water back-fills the freshwater tank through the water pump when you’re connected to water at the campsite. If it overflows, it does so to an external drain/vent on the freshwater tank.

        • The cause is what’s described in “what’s happening” above. It is your freshwater tank filling because the water pump is incorrectly allowing city water to back fill through it.

  4. I have both a city water connection and a fresh water connection. The fresh water is for the coach to use without the pump. The city water fills the water tank. I am getting water pushed out the fresh water connection when I use the pump that pulls only from the water tank.

    What is causing this? I suspect there is a backflow type vale someplace, but don’t know where.

    • The cause is what’s described in “what’s happening” above. It is your freshwater tank filling because the water pump is incorrectly allowing city water to back fill through it.

  5. We found clean water under our trailer floor. It has no odor, but has satiated the area . We thought we were hooked up directly to city water

  6. Thanks, this worked for me. I’ll share to the Nash RV owners FB page.

    This has happened to me before, usually my water pump sounds like a Vietnam-era chopper when the check valve is stuck open. Usually a good indicator.

  7. Full water tank, not hooked up to city water pump losing pressure and just runs for 3 to 4 sec. every 10 to 12 mins. been turning off pump manually and only turn it on when needed, so what do you think


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