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How to test this cap?

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Treebeard
Treebeard Member, Email Confirmation Posts: 24
in Gas Heating

Grundfos UPS 15-42 F

1000014316.jpg

Brown wires are connected to the cap. DMM can measure caps. Getting colder…

Comments

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 11,748
    edited December 2025

    There are two common ways to measure a run capacitor

    1. remove the brown wires and connect the brown wires to a meter that measures capacitance
    2. follow the instructions in this video to test it while the motor is running

    https://www.youtube.com/shorts/8K_5VmwCiAQ

    In the case of your circulator capacitor you take the amp reading from one of the brown wires while the pump is operating. If that number is too low to get an accurate amperage you can add ten wraps of wire around the ammeter jaw and divide your reading by 10.  

    So if you get 4.94 amps with 10 wraps then you are really getting 0.49 amps.  Now multiply that by 2652 and you get 1299.48  

    Next you measure the voltage at the 2 brown wires to see that that reading is.  If that happens to be 253 volts AC then you divide the 1299.48  by 253 and you get the actual uf reading of 5.1   If your cap is rated at 5 then you are within the tolerance.  Good cap.  

    Edward Young Retired

    After you make that expensive repair and you still have the same problem, What will you check next?

    HomerJSmith
  • HomerJSmith
    HomerJSmith Member Posts: 2,743
    edited December 2025

    Treebeard, I can see you are a man of few words. I'm kinda dumb and need a lot of information. What's the problem?

    Grundfos caps are polypropylene. They are very stable and I have never found one bad or out of tolerance. By bad I mean it tests open or shorted or exceeds its rated microfarads. But, they sell them so they must fail. I have 3 instruments to test caps, one that will do it live to the rated voltage while measuring the microfarads.

    You can get an inexpensive cap tester off Amazon, but if you think it may be bad you definitely want to test it.

    image.png

    Under the black LN G-rnd is a plug and terminal board that capacitor and wiring connects to. If you remove the torx screws you can unplug the cap and test it. You should test it disconnected from the windings for a proper test.

    These leads may help.

    https://s3.amazonaws.com/s3.supplyhouse.com/product_files/36612_1.pdf

    https://www.supplyhouse.com/Grundfos-595424-Terminal-Strip-Kit-for-UP15-10MF-115V

  • mattmia2
    mattmia2 Member Posts: 15,984

    depends on the exact construction. film and foil caps usually don't fail, metalized film caps fail all the time. enough exposure to moderate surges could do in either type too

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