CSLB C-20 HVAC ContractorHeating SystemsMedium
A residential heat pump system is operating in heating mode. The outdoor unit's fan motor is running, but the compressor is not engaging, and the system is blowing cool air indoors. After checking power, what is the next most probable component to troubleshoot?
- AThe compressor start capacitor.
- BThe indoor blower motor.
- CThe reversing valve solenoid.
- DThe defrost control board.
Show answer & explanationAnswer & explanation
Correct answer: A. The compressor start capacitor.
If the outdoor fan is running but the compressor is not, and the system is calling for heat (or cool), a common cause is a failed compressor start component, such as the start capacitor. The start capacitor provides the initial electrical boost for the compressor motor to begin operation.
Why the other options are wrong
- B. The indoor blower motor is likely operating if cool air is blowing, so it's not the primary issue for lack of heat.
- C. The reversing valve solenoid controls the direction of refrigerant flow. While it could cause no heat, the primary issue described is the compressor not running, which is typically a separate electrical issue.
- D. The defrost control board manages the defrost cycle. While a fault could impact heating, it's less likely to directly prevent the compressor from starting while the fan runs compared to a start capacitor issue.
Compressor Start Capacitor
An electrical component that stores and releases electrical energy to provide a momentary boost of current to the compressor motor, helping it overcome inertia and begin rotating.
- Essential for starting single-phase AC motors, including compressors.
- Failure often results in a 'humming' sound from the compressor without it starting.
- Common failure point due to heat and electrical stress.
Memory trick: Compressor's hum, but no pump, points to the start's slump.