Problem! Freshwater Tank Unexpectedly Filling When on City Water

Doug S • April 14, 2025

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.

Conclusion

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.

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