Capacitors are the devices that give your air conditioner the initial jolt of power it needs to start. They also keep the system running with a continuous power supply for the duration of the cooling cycle.
Most capacitors are housed in the outdoor condensing unit of an air conditioner or heat pump. However, since they’re high-voltage parts, you shouldn’t handle them if you’re not a licensed HVAC technician. We’ll explain more about what capacitors do and when you might need to call for repairs in this guide.
How Capacitors Work
Air conditioners are high-powered machines, and they need more energy to work than homes are wired to 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 that the device can handle without failing. Air conditioner capacitors are commonly rated for 370V, 440V, or 525V depending on system 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 normal to start.
- The unit shuts off at random.
- There’s a smoky or burning smell from the exterior air conditioning 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, including these:
- Age and wear: Like all parts, capacitors have a limited life span. Most last about 10 to 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 proceed to test it if you’re not a licensed professional. Capacitors are high-voltage and store electricity even when the system is powered off. Mishandling them could lead to electrocution and cause serious injuries.
A professional HVAC technician will use a multimeter to test how much charge the capacitor is holding. They’ll also 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 of the same type and rating.
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 life span, it might be better to replace the whole system. New air conditioners can be expensive, but will usually perform better and more cost-efficiently. They also tend to have better support and more available replacement parts if something fails.
Frequently Asked Questions
Can I replace an AC capacitor myself?
AC capacitors are high-voltage components and should only be tested and worked on by professional HVACR technicians. You shouldn’t replace the capacitor in your system yourself, as this could result in serious injury or even death.
How can I tell if the capacitor needs to be replaced?
There are many symptoms of a bad capacitor, including:
- AC makes a humming noise when running
- AC takes longer than normal to start
- AC shuts off at random
- Smokey or burning smell from exterior air conditioning components while AC runs
- No cold air blows throughout your house while the AC is running
How much does it cost to replace an AC capacitor?
The cost to replace your AC capacitor can range between $100 and $250, according to our research. Check out Today’s Homeowner’s Guide on the cost of AC capacitors to learn more about your specific system.
