Your air conditioning system relies on many mechanical, electrical, and electronic components to function correctly. Although many of its electronics may seem highly sophisticated and tricky to understand, the role of your system's capacitors is relatively straightforward. Any home air conditioning system contains several start-and-run capacitors that are essential to its operation.

It's easiest to think of capacitors as very short-term batteries. These components store energy passively, but they discharge their energy quickly once you disconnect the power. This behavior makes capacitors useful for smooth, consistent energy flow and also for storing more electricity than a system could typically supply. This latter role is where your air conditioner's start capacitors come into play.

The Function and Location of Air Conditioner Start Capacitors

Air conditioners use start capacitors to supply a "jolt" that can help their motors to get moving. In this way, the capacitor acts similarly to the starter in your car. Where the starter provides the initial juice to begin the combustion process, the start capacitors in your air conditioning system give you compressor and blower motors the kick they need to start moving.

Unlike the small capacitors you may find in a typical circuit board, the start capacitors inside your air conditioner are relatively large devices. In most air conditioning systems, you can find the start capacitor for the compressor on the outdoor condenser unit. You will usually need to remove the condenser unit housing to find the capacitor.

Note that your start capacitor stores very high, dangerous levels of voltage that can remain even after disconnecting the power. If you are not confident in your electrical abilities, do not attempt to diagnose or replace the capacitor yourself. If you have any doubts at all, contact an HVAC professional.

The Symptoms of a Failing Capacitor

Without a functional start capacitor, your air conditioner's compressor will be unable to overcome its initial resistance and typically will not start. You may hear a click or a hum when your compressor would normally engage, but otherwise, the system is likely to remain silent and dead. An audible humming or buzzing noise is often a telltale sign that a capacitor has failed.

Your air conditioner may also use a start capacitor to kickstart the motors for the outdoor condenser fan or your whole-house blower. In these cases, your condenser fan (the fan on top of your outdoor unit) will refuse to start. This fan should always operate while the compressor runs, so something is wrong if you don't see it spinning.

Likewise, a failing start capacitor for your blower motor will prevent your home's fan from running. If you hear a buzzing sound from near your blower unit and you get no air from your vents, then there's a good chance your blower motor's capacitor is a goner.

Contact a residential air conditioning repair contractor to learn more.