What Are Air Conditioner Capacitors?

Capacitors are the devices that give your air conditioner the initial jolt of power it needs to start. They also keep the AC system running with a continuous power supply for the duration of a cooling cycle. 

Most capacitors reside in an AC or heat pump’s outdoor condensing unit. However, because capacitors are high-voltage parts, you shouldn’t handle them unless you’re a licensed HVAC technician. Let’s look at what capacitors do and when you might need help to make AC system repairs.

How Capacitors Work

Air conditioners are high-powered machines, and they require more energy than a home’s wiring can provide. That’s where capacitors come in.

Each capacitor sandwiches an insulating ceramic layer between two metal plates. The plates collect energy, while the ceramic helps to keep that energy in temporary storage. When your air conditioner needs a large burst of power, the capacitor releases its stored energy, and the process repeats.

There are a few different types of capacitors, each of which has different responsibilities within your HVAC unit:

• Start capacitors: These capacitors provide the initial burst of energy to start the HVAC motor. They operate for only a few seconds at a time, then disconnect once the motor reaches 75% of its maximum speed.
• Run capacitors: These capacitors work continuously during your AC’s active cycle. They keep the motor running at the required torque and protect the system’s efficiency.
• Dual-run capacitors: Many modern AC units use dual-run capacitors that combine start and run functions into a single unit. They typically have one common terminal, one terminal for the fan motor, and one for the compressor motor.

Capacitor Voltage Ratings

A capacitor’s voltage rating indicates the maximum voltage a device can handle without failing. Manufacturers rate most AC capacitors for 370V, 440V, or 525V, depending on a system’s power requirements.

When you replace a capacitor, you’ll need to use one with the same or higher voltage compared to the original. Using a capacitor with a lower rating can lead to both power failures and safety hazards.

Signs of a Failing Capacitor

Some signs of a failing capacitor are easily detected, and others may take a bit longer to diagnose. You should know what your air conditioner normally looks, acts, and sounds like and watch for any differences over time.

Contact your HVAC company if you see any of these signs:

• Your air conditioner makes a different humming noise from usual as it runs.
• The unit takes longer than usual to start.
• The unit randomly shuts off.
• There’s a smoky or burning smell from the exterior AC unit while it’s running.
• No cold air blows through your home while the unit is running.

Why Capacitors Fail

There are a number of reasons why capacitors fail. They include:

• Age and wear: Like all parts, capacitors have a limited lifespan. Most last between 10 and 20 years, depending on usage and environmental factors.
• Heat exposure: An air conditioner exposed to direct sunlight or in a particularly hot climate may experience more frequent capacitor issues.
• Power surges: Electrical surges from lightning strikes or other sources can damage capacitors and other sensitive components.
• Poor ventilation: Inadequate airflow around an outdoor AC unit can lead to overheating and accelerate capacitor wear.

Testing and Replacing Capacitors

If you suspect that your AC capacitor is failing, don’t test it if you’re not a licensed professional. Capacitors are high-voltage devices, storing electricity even when the AC system is powered down. Mishandling a capacitor can lead to electrocution or cause serious injuries.

Licensed HVAC technicians will use a multimeter to test how much charge the capacitor is holding. They’ll look for cracks, bulging, and oozing liquid from the capacitor and circuit board. If they need to replace the capacitor, they’ll discharge it and install a new one.

Costs and Considerations

According to Angi, the cost to replace an AC capacitor can be between $100 and $250, including materials and labor. Your costs will vary based on the type of capacitor that broke and your air conditioner’s make and model. 

If your AC is nearing the end of its expected lifespan, it might be better to replace the whole system. New air conditioners can be expensive, but they will usually perform better and be more cost-efficient. They also tend to have better support and more available replacement parts if something fails.