Fixing a Grundfos CU301

Discussion in 'Pumps and Tanks Well Forum & Blog' started by SSmith, Jun 15, 2014.

  1. SSmith

    SSmith New Member

    Messages:
    6
    Location:
    Alberta, Canada
    I think that my Grundfos CU301 2-gallon tank is waterlogged, as the pump cuts in and out when running water.

    It would be greatly appreciated if someone could post the steps to de-waterlog my tank.


    Thanks
  2. craigpump

    craigpump Well-Known Member

    Messages:
    1,085
    Location:
    ct
    You throw those little buffer tanks away.

    Sounds like there is a malfunction with the control unit or the transducer.
  3. SSmith

    SSmith New Member

    Messages:
    6
    Location:
    Alberta, Canada
    This is a new control unit.....I would like to know if the tank is waterlogged, I want to try that first, so how can I do this.....
  4. craigpump

    craigpump Well-Known Member

    Messages:
    1,085
    Location:
    ct
    Take a small screw driver and push down on the core in the snifter valve on the tank. If water comes out, the bladder or diaphram is bad.

    Those tanks need to have a precharge of 70% of shut off pressure. 70 psi shut off would be 49psi precharge.
  5. valveman

    valveman Moderator Staff Member

    Messages:
    4,586
    Location:
    Lubbock, Texas
    Even if the bladder in the tank is busted and the tank is waterlogged, that controller should hold a steady pressure while you are using water. However, when those CU301 controllers malfunction they revert to cycling on and off like a pressure switch with 15 PSI between on and off. If the bladder in the tank was not busted, it will be after the CU301 bounces the pump on and off like that for a short while.

    It doesn’t matter that the CU301 is new. When they lose communication with the electronics in the motor, they begin to cycle the pump on and off as a default. Because they use the same wire that powers the pump for the communication signal as well, this lose of communication is a common occurrence.
  6. SSmith

    SSmith New Member

    Messages:
    6
    Location:
    Alberta, Canada
    I pressed in on the snifter valve and no water came out, so that must rule out the bladder being bad......

    So what do I need to do now to get the tank up to 49psi precharge, we live out in the boonies so I am going to have to do this myself....

    Thanks
  7. Reach4

    Reach4 Well-Known Member

    Messages:
    2,814
    Location:
    IL
    To set the precharge on a pressure tank, turn off the pump. Release the water pressure by opening a valve. Add with a tire pump, or remove air from the Schrader valve, and measure the pressure with a tire gauge. I guess it would be good to have an accurate tire gauge.

    Do not measure or change the air pressure when you have water pressure. When done, put the cap on the Schrader valve, and turn on the pump.

    I have not used your kind of pump, but setting the precharge method is going to be the same.
  8. mliu

    mliu Member

    Messages:
    101
    Location:
    California
    Did you just depress the valve very quickly or did you let a good blast of air out? Even if the bladder is bad, there could still be an air bubble trapped in the tank, so if you just let a squirt of air out, you may not know the real condition of the bladder. Recharging the tank is simple so you might as well do a solid test by letting out a lot of air. It should be completely dry with no water spray or spurting.
  9. valveman

    valveman Moderator Staff Member

    Messages:
    4,586
    Location:
    Lubbock, Texas

    It sounds like your CU301 has lost communication with the electronics in the motor. I think the latest version (generation 7) will cycle like that 1000 times before it tells you it has lost commincation.
  10. SSmith

    SSmith New Member

    Messages:
    6
    Location:
    Alberta, Canada
    Thanks guys, added the air pressure to the tank and no more surging......much appreciated
  11. valveman

    valveman Moderator Staff Member

    Messages:
    4,586
    Location:
    Lubbock, Texas
    Or after turning on and off (rebooting) the motor finally picked up communication with the controller.

    Glad you got it working.
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