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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?
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.
