Like most electrical motors, your air conditioner's compressor must draw extra power from your home's supply when it starts up. It's easy to see this happening by watching for dimming lights whenever your air conditioner turns on. This behavior is normal and expected, but it's still an issue that engineers must consider when they design air conditioning systems. 

One common way to address this problem is by using a start capacitor. A capacitor is a simple electronic component that stores and releases energy. In this case, the capacitor helps provide the extra jolt your compressor needs to get going. Start capacitors are a relatively common failure point, but it's important to understand why the capacitor failed before installing a new one.

How Does the Start Capacitor Work?

All capacitors generate heat due to electrical resistance. The longer that current runs through a capacitor, the more heat it generates. Capacitors use many designs to help dissipate heat, depending on their intended applications and how long the capacitor will need to remain in the circuit. Since start capacitors only remain in a circuit momentarily, they don't need to worry much about heat dissipation.

Of course, you don't physically remove your capacitor each time your air conditioner starts. Instead, manufacturers use a potential relay to remove the capacitor from the circuit. Once the compressor gets up to speed, the relay opens the connection to the start capacitor and disconnects it from the circuit. This whole process may take less than a second.

What Happens When It Fails?

Unsurprisingly, a failed start capacitor will cause your system to struggle to start. You may hear your compressor taking much longer to turn on, or it may not be able to turn on at all. You may also notice warm or humid air blowing from your vents if the blower turns on, but the compressor fails to get started.

If you know how to open your AC unit, you can usually visually inspect the capacitor to look for signs of failure. These indications include leaking fluid, a damaged top, or other relatively clear indications of something wrong. A professional will confirm the problem by checking the capacitance with a multimeter.

Why Shouldn't You Immediately Replace Your Capacitor?

Recall that the start capacitor only briefly remains in the circuit. As a result, it doesn't have much time to generate heat or even suffer if ambient conditions are too hot. In other words, start capacitors rarely fail on their own. Instead, they typically fail when another problem, such as a faulty potential relay, causes them to remain in series for too long, ultimately generating too much heat and failing.

If you blindly replace your start capacitor without investigating the underlying cause, your new one may fail quickly. While replacing the capacitor may seem fairly straightforward, contacting a professional is always better. An HVAC expert will help you get to the bottom of the problem to keep your new capacitor from joining your old one in the trash heap.

Contact a local HVAC service to learn more about AC repair.