Why Did Your AC's Start Capacitor Die?

Posted on: 19 December 2022

Start capacitors are a relatively common replacement part on many air conditioning systems. This electrical component stores the energy that the compressor needs to start running. Since the compressor draws the most current on startup, the capacitor must help deliver a smooth and consistent power supply.

As a result, a failed start capacitor can cause your system to struggle or stop working altogether. It's often easy to diagnose start capacitor failures since there are often visible signs of trouble. However, does that mean you should replace your capacitor and call it a day? In most cases, the answer is "no." Understanding why these components fail is critical to avoid repeated failures.

The Difference Between Start And Run Capacitors

While nearly all air conditioning systems include start capacitors, not all air conditioners will include run capacitors. These two components are relatively similar, but they have very different functions. The start capacitor stores energy to provide a quick burst to the compressor, and its design only requires it to remain in the circuit for a brief moment.

On the other hand, a run capacitor helps to smooth power delivery as the compressor runs. This capacitor remains in the circuit the entire time that your compressor operates. Since the run capacitor operates for much longer, it's far better at dissipating heat than the start capacitor. Conversely, the start capacitor can overheat and fail relatively quickly.

Common Causes for Start Capacitor Failures

Like any electrical component, the start capacitor in your air conditioner will eventually fail due to age. However, capacitors can also fail due to excessive heat. In the case of a start capacitor, the most likely culprit for excessive heat generation is an electrical failure elsewhere in the system. In most cases, any problem that forces the start capacitor to remain in the circuit for too long will cause premature failure.

The most common cause of this issue is a damaged relay. The relay briefly brings the start capacitor into the compressor circuit, allowing it to provide the necessary starting energy before pulling it back out of the circuit. A fused relay may remain in the closed position, allowing the capacitor to remain in the circuit indefinitely. This situation will typically cause the start capacitor to fail catastrophically.

If you attempt to replace your start capacitor blindly, you may find that your new capacitor fails immediately or in relatively short order. Always have a trained HVAC technician examine and diagnose your system to avoid this situation. The added cost of a professional visit will ensure your system continues to function safely and prevent you from needlessly replacing the same part repeatedly.

For more information on air conditioner repair, contact a professional near you.


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