Serviceman checking AC capacitor

Why Is My AC Not Running? It May Be Your Capacitor

Home air conditioner capacitor replacement is one of the top electrical repairs Air Zero makes on a daily basis. This simple component can take the entire HVAC system offline and can create additional issues within the system if not quickly repaired.

What Is a Capacitor? How Does a Capacitor Work?

An AC unit capacitor stores an electrical charge momentarily then releases it back into the electrical circuit to correct the power factor, altering the magnetic field of the motor that it is wired to and making it more efficient.

The capacitor required for a motor is specified by the manufacturer. Capacitors used in air conditioning units are rated in microfarad, which is a rating of capacitance.

Types Of Capacitors

  • Run Capacitor
    The purpose of the run capacitor is to help the motor run more efficiently. A run capacitor stays in the circuit the entire time a motor is operating. In a central HVAC system, this typically includes the outdoor condenser fan motor, the compressor, and the blower motor in the air handler inside. 

Run Capacitor

AC Run Capacitor Example

Dual Run Capacitor
  • The condensing unit outside commonly has a dual run capacitor. This HVAC capacitor has two different microfarad ratings built into a single capacitor, separated internally. This capacitor provides capacitance to the AC compressor and the condenser fan motor.

AC Dual Run capacitor example

  • Start Capacitor
    The purpose of a start capacitor is to give a quick electrical boost to the compressor. The most strain electrically on the compressor is at startup. The start capacitor helps overcome the initial mechanical inertia but then drops out of the circuit within a fraction of a second.

    The microfarad rating of start capacitors tends to be much higher. Not all systems have a start capacitor. Sometimes if a compressor is having a difficult time starting, a hard start kit/ start capacitor with relay ( same thing ) can be added to the system to help extend the life of the compressor.

Start Capacitor with relay

AC Start capacitor with relay

Symptoms Of a Bad HVAC Capacitor

  • Outdoor Fan Is Running Slow Or Not At All
    If you notice the outside unit ( condenser unit ) is not spinning, as usual, it may be indicative of a weak or failing run capacitor. If you take a stick and spin the fan blade and the condenser fan starts to run, the run capacitor has likely failed.

    Even though the fan is operating, we recommend that the AC unit should be shut off. Without the run capacitor in the circuit, the motor will pull excess amperage. This can damage the motor and lead to a more expensive repair.

  • The Capacitor Is Bulging
    If the capacitor is swelling on the top ( has a domed appearance), excess pressure has built up internally and this is a sign the air conditioning capacitor is defective and needs to be replaced.

AC Run capacitor swollen

  • The Capacitor Has Dielectric Oil On It
    Dielectric oil is used in capacitors to provide cooling and as an electrical insulator. If you notice an oily film in the electrical cabinet or on the capacitor itself, it may have failed.

    Sometimes pressure builds up, and the dielectric fluid from the capacitor seeps out. A sure sign the capacitor has failed. Sometimes the capacitor will explode from the internal pressure.

  • Wiring Terminals Are Burned Off
    If you notice the top of the cap has burned wiring or corroded electrical terminals, it is a problem. Sometimes this results from excess amperage from a defective AC capacitor, motor, or a poor electrical connection.

Wiring Burnt off Dual capacitor

Wiring Burnt off AC dual run capacitor

What Causes AC Capacitors To Fail?

  • A Variance In Voltage
    The incoming voltage to your house constantly varies. When the voltage is lower than required, an electric motor pulls more amperage.

    A sudden surge of high voltage can be detrimental too. The run/start capacitor tends to be the weak link in the circuit and fails.

  • Motor Failure
    If a condenser fan motor, blower motor, or compressor begins to fail (bearings) or seize up entirely, the amperage on the circuit skyrockets and can wipe out the capacitor.

    If the air conditioning unit has a start capacitor and the compressor seizes up. The start capacitor may not drop out of the circuit and stays energized, and failure can occur.

Air Zero Sells Run & Start Capacitors