What Are HVAC Start and Run Capacitors and How Can a Malfunction Stop Your System in Its Tracks?

If your air-conditioning system could select a most valuable player, the compressor might win the title. The motor-driven compressor sits at the start of the cooling process and compresses gas refrigerant out into the system to provide the fuel for your indoor climate change. If the compressor stops working correctly, your entire HVAC system will suffer.

Compressors can suddenly start experiencing problems starting up or staying running for extended periods of time. These problems tend to trace back to two related parts: the start capacitor and the run capacitor.

What is the HVAC start and run capacitor, how can you diagnose this problem, and how can an air-conditioning services company help?

Defining Start and Run Capacitors

The capacitors are cylindrical electricity storage devices that reside near the compressor in the condensing unit. Not every unit has a start capacitor, so you will need to consult your owner's manual to know whether this capacitor is even present.

If you do have both, the start capacitor provides an energy boost to help the compressor get going once you turn on your thermostat while the run capacitor helps keep the compressor operating smoothly. The capacitors can counteract glitches in the electrical supply that could cause the compressor to shut down at a critical moment, and that would also effectively turn off your air conditioner.

Diagnosing the Capacitors

Capacitors don't have any moving parts. The only problems that can occur are the attached wires going bad or, more commonly, the capacitor itself failing. You can check for these problems yourself if you own a multimeter that has both AC and Ohms settings.

You will need to turn off the circuit breaker for your air conditioner, but remember that the capacitors will still store some charge, so use caution while working. You can drain the charge out before testing. On the start capacitor, just remove the wires, hook the multimeter probes to the terminals, turn the meter to AC, and wait for the number to zero out. On the run capacitor, you want to hold the end of an insulated screwdriver across the terminals before checking the drain with the multimeter on AC.

Now you can test each capacitor using the multimeter set to Ohms and the numbers printed on the side of each part as a reference guide. If the capacitor reading falls within that number range, the part is healthy. If the reading is higher or lower than the range or keeps jumping around, then you likely need a new part.

Replacing the Capacitors

You want to leave the replacement of the capacitors in the capable hands of an air-conditioning services company such as Smedley & Associates or an appliance-repair service. Making a mistake during installation can cause irreversible damage, and you don't want to create even more problems for yourself.