AC Fan Running but Compressor Not

How the Fan and Compressor Run Together

Your outdoor condensing unit contains two main electric motors: the outdoor fan motor and the compressor. When your thermostat calls for cooling, it sends a 24 V control signal from a transformer to the contactor coil. The coil pulls the contactor points closed, allowing 240 V line power to reach both parts at the same time.

Because both parts receive power through the contactor, a running fan proves that the contactor has pulled in and 240 V power is reaching the outdoor unit. If the fan spins smoothly but the compressor remains completely still or only makes a quiet hum, the electrical supply split to the compressor has been interrupted, or the compressor motor cannot start.

The HERM Side of a Dual Run Capacitor

Most residential outdoor units use a dual run capacitor. A dual run capacitor powers two separate motors from a single metal can. It features three sets of terminals on top: HERM, FAN, and C.

The C terminal stands for common and connects to incoming power. The FAN terminal supplies the phase boost to keep the outdoor fan spinning. The HERM terminal, short for hermetic compressor, connects directly to the compressor start winding.

A dual run capacitor can fail on just one side. The FAN side might still function properly, allowing the fan blades to turn at full speed. At the same time, the HERM side may drop below its rated microfarad rating or fail completely. When the HERM side fails, the compressor does not receive the electrical boost it needs to turn over and start.

• C terminal: Common connection for incoming power.
• FAN terminal: Supplies the outdoor fan motor.
• HERM terminal: Supplies the hermetic compressor motor.

Inspecting and Matching a Replacement Capacitor

When a capacitor fails, it often shows physical signs of damage. A swollen top, a domed bottom, or leaking oil are clear signs of failure. However, a capacitor can also fail with no visible changes on the outside.

If you choose to replace a dual run capacitor, the ratings must match your old part. Capacitance is measured in microfarads, written as µF, MFD, or uF. A dual capacitor lists two numbers, such as 40 + 5 µF. The larger number belongs to the HERM terminal, while the smaller number belongs to the FAN terminal.

The voltage rating on the new part must be the same or higher. For example, a 440 V capacitor can replace a 370 V capacitor, but a 370 V capacitor can never replace a 440 V part. You can choose either a round or oval shape as long as it fits inside the mounting bracket on your unit.

1. Turn off power at the main breaker and pull the outdoor disconnect.
2. Remove the electrical access panel on the outdoor condenser.
3. Test for voltage to verify that power is fully disconnected.
4. Discharge the old capacitor using an insulated tool across the terminals.
5. Take a clear photo of every wire connected to C, FAN, and HERM.
6. Move wires one at a time to the matching terminals on the replacement capacitor.

Hard Start Kits for Struggling Compressors

In some situations, a compressor is mechanically sound but struggles to overcome high pressure or friction during startup. This is where a hard start kit can help.

A hard start kit connects to the run capacitor wiring. It contains a start capacitor and a relay. A start capacitor provides a high burst of capacitance, often shown as a range such as 88 to 108 µF. The kit gives the compressor motor extra starting torque for a fraction of a second, and then the relay drops the start capacitor out of the circuit once the motor is running.

Hard start kits like the TorQstart TQS6 or Supco SPP6 offer a 500% torque multiplier to help free a tight motor. However, most units start normally without one. You should always check and replace a weak dual run capacitor first before deciding to install a hard start kit.

Overheated Compressor Thermal Overload Resets

Compressors generate significant heat while pumping refrigerant. To prevent permanent motor burnouts, compressors contain an internal thermal overload switch. This safety device opens the electrical circuit if the motor gets too hot.

If the run capacitor is failing, the compressor may try to start, pull high electrical current, and heat up rapidly without spinning. The thermal overload switch will trip to protect the windings. Once tripped, the compressor will not turn on even if power is present, while the fan continues to run.

An internal thermal switch resets automatically, but it requires the compressor to cool down first. Cooling an overheated compressor can take several hours. If the compressor shell feels extremely hot to the touch, turn the system off at the thermostat and breaker to let it cool completely before testing it again.

Checking the Outdoor Contactor

The contactor is the electrical switch that routes 240 V line power to the outdoor components. Contactors come in 1 pole and 2 pole configurations, commonly rated for 24 V coil voltage and 25 to 40 amps.

Over years of service, the electrical contact points can become pitted, scorched, or covered in carbon from electrical arcing. On a 1 pole contactor, one line remains connected while the other is switched. If the switched contact point is badly burned, it may supply erratic power or fail to pass full voltage under load.

Inspect the contact points for heavy black pitting or melted plastic. If the contacts are damaged or buzzing loudly, the contactor must be replaced with one matching the pole count, coil voltage, and equal or higher amperage.