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I understand the purpose of a run capacitor. And I understand the purpose of a start capacitor. I understand the concept of a dual capacitor.

With respect to HVAC condenser unit dual capacitors which are labeled "HERM" "COM" and "FAN": Is the purpose of the HERM capacitor for run or for start? Also, is the purpose of the FAN capacitor for run or for start?

  • Generally they are used as the run capacitor (HERM is for the hermetically sealed compressor, Fan is obviously the fan and COM is the Common conductor. ) although it will not matter what you were to use it for. A capacitor is still a capacitor (only the dual is a two in one design) reduces part count. A manufacturer can use a dual capacitor in any way they like - start or run or even mix and match that. – Ken Apr 13 '18 at 22:31
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A dual run capacitor is always a run capacitor. HERM is the run capacitor for the compressor and fan is the run capacitor for the fan. Common completes the circuit to both. They are essentially 2 separate capacitors and could be split into 2 capacitors at a later date if you couldn’t find the right dual capacitor. Dual capacitors are usually used to save space and cost of manufacturing. Not all air conditioners have start capacitors. Depending on whether or not the engineer feels a start capacitors necessary will determine if yours has a start capacitor or not. Since a start capacitor can only stay in the circuit for a split second without burning out the motor it will always be attached to a device to take it out of the circuit like a potential relay or a solid state relay. The only time I ever add a start capacitor to a system is if the bearings are tight and the start capacitor is a stop gap measure before getting a new compressor but I let the owner know there are no guarantees. I have had some last years this way and others fail within a day. I have also installed a few when they were dimming the lights significantly on startup but this was only after conferring with the manufacturer first.

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