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How often should Capacitor be tested?

I am wondering how often one should test the Heat Pump dual run capacitor?
(mine is 45+5MFD 440VAC)

Every year? Every 2 years?

And what percent drop in microFarads should be considered replacement time?
10 percent? 6 percent?



  • Big Ed_4
    Big Ed_4 Member Posts: 2,785
    With 5% is acceptable .. I only check them is the motor or compressor is not working
    I have enough experience to know , that I dont know it all
  • SlamDunk
    SlamDunk Member Posts: 1,580
    What @Big Ed_4 Said.

    But they go for 10-15 bucks on amazon if you want to be proactive.
    EBEBRATT-Ed Member Posts: 15,519
    And by a USA capacitor. The China ones ar no good they don't last
  • HowardZ
    HowardZ Member Posts: 15
    Is Titan-HD a good model ?
  • mattmia2
    mattmia2 Member Posts: 9,637

    And by a USA capacitor. The China ones ar no good they don't last

    Mexico is probably ok too.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,834
    I asked my technicians to test the caps on every tune-up. It is easy to do a capacitor test when the motor and the compressor are operating. There is no need to remove wires that may result in damaging a perfectly good capacitor. I heard this way was more accurate too.

    Here are the steps:

    1. Measure the amperage of just the start wire (wiring connecting to the start winding). This will be the wire between the capacitor and the compressor. In the case of 4-wire fan motors, it will usually be the brown wire — not the white-striped brown wire. In the case of a compressor with a dual capacitor, it will be the wire going to the HERM terminal. Note your amperage on this wire.

    2. Now, take the amp reading you took on the start wire (wire from the capacitor), and multiply by 2,652 (some say 2,650, some 2,653, and some 2,654, but 2,652 is perfectly accurate). I remember 2,652 because 26 doubled equals 52.

    3. Next, measure voltage across the capacitor. For a compressor that would be between HERM and C, this is the measured voltage across the start and run terminals on the motor.

    4. Divide the total of the start wire amps times 2,652 by the voltage you just measured. This total is the capacitance. The complete formula is:

    Start Winding Amps x 2,652 ÷ capacitor voltage = microfarads.

    5. Read the nameplate MFD on the capacitors and compare to your actual readings. Many capacitors allow for a 6 percent +/- tolerance. If outside of that range, then replacement of the capacitor may be recommended. Always double check your math before you quote a customer.

    6. Repeat this process on all of the run capacitors and you will have assurance whether they are fully functional under load or not.

    7. Keep in mind that the capacitor currently installed may not be the correct capacitor. The motor or compressor may have been replaced, or someone may have put in the wrong size. When in doubt, refer to the data plate or specs on the specific motor or compressor.

    So, let’s say you read a start winding amperage of 4.4 x 2,652 = 11,668.8 divided by a measured voltage of 335 V = 34.83 μF.

    If the run capacitor was rated at 35 μF, this would be within range.

    If it was rated at 40 μF, it would be 15 percent out of range, which is outside of the allowable range for all run capacitors I know of.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • HowardZ
    HowardZ Member Posts: 15
    I have the Fluke 116. It can measure capacitance, but for amperage only milliamps, and it did not come with a clamp for measuring Amps.
    Which Fluke model do your technicians use?
  • mattmia2
    mattmia2 Member Posts: 9,637
    You can get a clamp on current probe for that meter or a separate clamp on ammeter. I have never found clamp on ammeters to be super precise.

    I would give it 20% or more before condemning a capacitor. I think a bad or failing cap is going to be way off, not just a little off.
    HVACNUT Member Posts: 5,833
    I check caps at every maintenance.