You've probably heard about your AC's start capacitor. Nearly all residential air conditioners use a start capacitor to provide the initial power necessary to get the compressor moving. This capacitor stores energy while the capacitor is off, preventing an excessive load on your home's electrical system and minimizing the likelihood of a tripped breaker.

However, some air conditioners use two capacitors. This second capacitor, known as a run capacitor, serves a much different purpose. Unlike a start capacitor, the run capacitor remains in the circuit the whole time your compressor runs. The run capacitor "smooths" power delivery to the compressor, ensuring it can run efficiently and consistently.

Why Do Run Capacitors Fail?

If you're familiar with automotive or marine batteries, you know they typically come in two flavors: start and deep-cycle. Start batteries provide a quick jolt of power, while deep-cycle batteries provide consistent power over time. Start and run capacitors have a similar distinction. A start capacitor discharges quickly, while a run capacitor remains in the circuit longer.

Start capacitors typically fail because of another problem causing them to remain in the circuit for too long, such as a stuck relay. On the other hand, run capacitors are more likely to fail due to normal wear and tear. Since the capacitor continuously remains in operation, the heat can eventually overwhelm the capacity of its internal oil to keep the metal sheets inside cool.

Run capacitor failures are often relatively obvious on visual inspection. In many cases, you'll notice a swollen casing or even oil visibly leaking from the capacitor. However, capacitors can fail more silently, so it's important always to have a technician use proper testing equipment to confirm that your capacitor is at fault.

What Happens When Your Run Capacitor Fails?

A faulty start capacitor will typically stop your compressor from engaging, so your air conditioning system won't work at all. However, a faulty run capacitor can produce some additional symptoms. You may notice problems such as your air conditioner's compressor being unusually noisy or producing noticeable vibrations.

Note that a run capacitor can stop your system from working. Since the compressor connects to the capacitor, a total failure will prevent it from receiving the voltage it needs to operate. As a result, you may notice your outdoor unit's condenser fan running but not hear your compressor turn on at all. The compressor may also briefly engage and then quickly shut off.

Although capacitors are relatively cheap and easy to replace, they're critical parts of your air conditioning system. More importantly, they connect to the most essential and expensive component in any air conditioning unit: the compressor. Because of the potential for a faulty capacitor to damage your compressor, it's important to rely on a pro for diagnosis and repair.

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