AC Capacitor Replacement (2024)

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Safety Precautions for Replacing AC Capacitors

AC capacitors can be dangerous if not handled correctly. However, preparation and proper safety equipment ensure the process is safe and efficient for HVAC technicians.

You will need to following equipment to safely replace an AC capacitor:

• Safety glasses
• Electrician’s work gloves
• Multimeter
• Insulated tools, including a screwdriver and needle nose pliers
• A set of small jumper cables with a resistor

Before troubleshooting or replacing a capacitor, make sure the power supply to the AC unit is shut off completely. Then locate the side panel on the AC unit and remove it to continue working.

• Turn the thermostat off.
• Turn off the main power supply to the breaker connected to the AC unit. This is typically close to the AC compressor.
• Remove the circuit breaker from the breaker box and set it aside.
• Examine the capacitor for any signs of damage or malfunction. Broken capacitors typically bulge up from the top and their terminals point in different directions.
• Set the capacitor on a flat surface to ensure it is not bulging from the bottom. If the capacitor rocks back and forth, it is time to replace the capacitor.

Capacitors must be discharged before testing and troubleshooting can begin. There are two common methods of discharging capacitors. One requires an insulated metal tool like a screwdriver, and the other requires a set of jumper cables attached to a resistor.

Make a note of where each wire is connected on the capacitor. Every capacitor has a common (C) terminal. Single capacitors also have either a condenser fan (FAN) or a hermetically sealed compressor (HERM) terminal. Dual capacitors have all three of these terminals.

• The screwdriver method requires connecting the screwdriver to the C terminal and either the FAN or HERM terminal. If there is a charge, the capacitor will release it, and the screwdriver will vibrate quickly with a buzzing sound. Be sure to repeat this process between the C terminal and either the FAN or HERM terminals if you are working with a dual capacitor.
• The jumper cable method requires a pair of jumper cables that are connected to a resistor. Connect one jumper to the C terminal, and the other to either the FAN or HERM terminal. Any charge in the capacitor will discharge through to the resistor.

If possible, discharge the capacitor before removing any wires. This provides the most safety when removing the capacitor from the AC unit. However, the wire fittings may obstruct a clear path between the terminals for a screwdriver to connect. If you do not have jumper cables and a resistor, use needle nose pliers to detach the wires and clear the way for your screwdriver.

Once the capacitor is discharged, you can test its capacitance and resistance with a multimeter. First, set your multimeter to the capacitance function.

• To test the condenser fan section of the capacitor, attach one of the multimeter jumpers to the condenser fan (FAN) terminal, and the other to the common (C) terminal.
• To test the compressor fan section of the capacitor, attach one of the multimeter jumpers to the hermetically-sealed compressor (HERM) terminal, and the other to the common (C) terminal.
• For both sections, wait for the capacitance reading to appear on the screen. A good reading will display the microfarads within 10% of the amount specified on the label.

Another option for testing capacitors is by measuring its resistance. An analog multimeter is recommended for this because digital multimeters do not always show the changes in ohms that indicate a working capacitor. Set your multimeter to the resistance (ohms) function.

• Attach one of the jumpers to the C terminal and the other on the FAN terminal.
• The needle should jump up and then fall back down to infinity ohms. If it does not, the capacitor may need to be replaced.
• Reverse the jumpers and look for the same response from the needle.
• Repeat this process between the C and HERM fan terminals.
• Attach one of the jumpers to the C terminal, and touch the other to the side of the capacitor. If the reading indicates continuity by showing very low or zero ohms, the capacitor must be replaced.

Pro Tip: For the most accurate assessments, bring both digital and analog multimeters to the job site.

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How to Find the Right Replacement Capacitor

A replacement capacitor must meet the specifications listed on the old capacitor. The label will contain the information required to ensure a new part is compatible with the AC unit. Some capacitors are made to fit many types of units, but most are manufactured for a specific purpose. When selecting a new capacitor to replace a broken one, ensure the following specifications match:

• Capacitance: Measured in microfarads, (µF or MFD) the capacitance refers to how much charge the capacitor can hold.
• Voltage: Capacitors are usually rated for both 370 volts and 440 volts. You can use a new capacitor with a higher voltage than the original, but never choose one that has a lower voltage rating than the system voltage.
• Frequency: AC capacitors are typically rated for 50 and 60 hertz. If the only accessible capacitors are rated for just one, ensure it is rated for the same frequency as the original capacitor.
• Terminal Types: Capacitor terminals include one to four metal tabs. Make sure that your replacement capacitor has terminals with enough tabs to support all necessary connections to reassemble the AC unit.
• Case Shape: Capacitors will typically resemble a round cylinder or an oval shape. If the capacitance, voltage and frequency are consistent with the old capacitor, the new capacitor can be of either shape.

When using capacitors with more than three terminals, follow the instructions included with the capacitor to ensure you connect the wires properly for an AC unit. For some units, two of the terminals may need to be connected by a wire in addition to the AC unit itself.

Pro Tip: The average AC capacitor lasts about 20 years. However, prolonged humidity and hot weather can add wear and tear. In areas like Florida or Texas, you can expect to replace a capacitor after about 15 years.

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AC Capacitor Replacement Procedure

A capacitor must be discharged before it can be replaced to ensure safe disposal. Once the old capacitor is discharged, continue removing it from the AC unit and prepare to install the new one:

• Disconnect the wires from the capacitor.
• Use a multimeter to test the capacitance of each terminal. Connect one prong to the C terminal and the other to either the FAN or HERM terminal. A broken capacitor will read very low compared to the capacitance printed on its label. Dual capacitors must be replaced even if only one of these terminals is not displaying the proper capacitance.
• Unscrew the metal strap holding the capacitor in place and remove the capacitor from the AC unit.
• Take your new capacitor out of the box and affix the metal strap around it so that it is affixed to the AC unit.
• Attach each wire to the new capacitor in the same configuration as the old capacitor.
• Return the breaker to the breaker box, turn the main power on, and turn on the thermostat
• Ensure the AC unit is working normally again. If you replaced the capacitor properly and the AC unit is still running slow or not dispensing cool air, there may be another problem.

Pro Tip: If the metal strap from the old capacitor does not fit the new capacitor, cut some pipe hanger strapto length. Screw it into the same hole where the original strap was connected.

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