What is An Air Conditioning Capacitor?

Ever since you became a proud owner of a central air conditioning system, you’ve known it as one of your home’s “mechanicals.” When you’re out looking for a home, you want the mechanicals to be inspected before you agree on a purchase price. If your AC system needs attention, you contact a mechanical service company for assistance.

And yet with all the emphasis on mechanical this and mechanical that, it’s just as

Important to remember that AC and heating systems can’t function without electricity.

Case in point

Your AC system is built with electrical capacitors. The job of a capacitor is to store up electricity, kind of like a rechargeable battery, so that it can supply a small burst of energy to the motor it’s connected to when needed. There are two basic kinds of capacitors found in an HVAC unit: the start capacitors and the run capacitors. The start capacitor provides the extra voltage needed to get the compressor or fan motor start, while the run capacitor provides energy to keep them running.

Air Conditioning Capacitor

If your start capacitor has failed, then you might hear your system humming without starting up. If your run capacitor is failing, then you might hear your system start up, but then it will cycle off before it should—a process called short-cycling.


A weak capacitor attached to the compressor could cause what is known as “hard starting”. This is when the AC struggles to turn on and then shuts off again soon after. Hard starting puts an enormous strain on the compressor. And when a compressor starts to fail, the AC system will need professional attention immediately.

To help keep your air conditioner from overheating, set the thermostat a little higher on those hot days. Take other steps to improve your energy efficiency, like using reflective blinds on east- and west-facing windows.

Power surges can cause a capacitor to fail, too. While a lightning strike can overload and burn out the unit’s electrical systems, milder power surges can cause capacitors to fail over time. Right there you have a very good reason to acquire a whole-house surge protector.

Much like a rechargeable battery, a capacitor’s ability to store and release energy decreases over time. It’s inevitable that capacitors will eventually wear out. The best way to keep capacitors working as well as possible and for as long as possible is with annual preventive maintenance. During our tune-up and inspection process, Air Professionals checks all system components, including the capacitors. That’s the best time to replace a failing system component – not when it’s already failed.

For the best in season-long AC performance, contact Air Professionals today and ask for our preventive maintenance service.

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