RSES Troubleshooting 5 — Questions and Answers
Question 1: A rooftop packaged unit condenser fan motor fails in summer. What will happen to system operation if ignored?
- Suction pressure will drop causing the low-pressure switch to trip
- Head pressure will rise, potentially tripping the high-pressure switch or damaging the compressor (Correct answer)
- Subcooling will increase, improving system efficiency
- Refrigerant will migrate to the evaporator coil
Correct answer: Head pressure will rise, potentially tripping the high-pressure switch or damaging the compressor
Without condenser airflow, heat rejection fails, causing discharge pressure to rise until the high-pressure cutout trips or the system sustains damage.
Question 2: A technician recovers refrigerant from a system, then pulls a deep vacuum and the vacuum rises rapidly when the pump is isolated. What does this indicate?
- The system is clean and dry
- There is a leak or residual moisture in the system (Correct answer)
- The vacuum pump reached its maximum performance
- The refrigerant has been fully recovered
Correct answer: There is a leak or residual moisture in the system
A rising vacuum after pump isolation indicates either a system leak (air entering) or moisture boiling off, both requiring remediation before recharge.
Question 3: What is the significance of measuring temperature difference (TD) across an evaporator coil during troubleshooting?
- TD indicates the refrigerant type in the system
- TD measures airflow adequacy and coil heat transfer effectiveness (Correct answer)
- TD directly measures refrigerant superheat
- TD is used to calculate condenser subcooling
Correct answer: TD measures airflow adequacy and coil heat transfer effectiveness
The temperature difference between air entering and leaving the evaporator reflects airflow volume and coil efficiency; abnormal TD suggests airflow or coil problems.
Question 4: A refrigeration system uses a capillary tube metering device and is suspected of being undercharged. How is the charge typically verified on this type of system?
- By measuring subcooling at the condenser outlet
- By weighing in the exact factory charge after a complete evacuation (Correct answer)
- By measuring superheat at the evaporator outlet
- By checking high-side pressure only
Correct answer: By weighing in the exact factory charge after a complete evacuation
Capillary tube systems are critically charge-sensitive and are best charged by weighing the exact factory specification into an evacuated system.
Question 5: During troubleshooting, a technician finds that the contactor coil is energized but the compressor does not start. The compressor hums briefly then the internal overload trips. What is the most likely cause?
- Faulty low-pressure control
- Single-phasing (loss of one leg of power) or failed start components (Correct answer)
- Refrigerant overcharge locking the compressor
- Defective reversing valve
Correct answer: Single-phasing (loss of one leg of power) or failed start components
A compressor that hums but fails to start and trips its overload is typically experiencing single-phasing or has failed start capacitors/relays preventing proper motor starting.
Question 6: A technician suspects a hermetic compressor has an internal motor winding short. Which instrument confirms this diagnosis?
- Manifold gauge set
- Clamp-on ammeter only
- Megohmmeter (megger) or ohmmeter measuring winding resistance (Correct answer)
- Electronic leak detector
Correct answer: Megohmmeter (megger) or ohmmeter measuring winding resistance
A megohmmeter measures insulation resistance between windings and ground, confirming a winding short or ground fault in a hermetic compressor motor.
Question 7: What condition is indicated when a system's sight glass shows clear liquid but system pressures are abnormal (low suction, high discharge)?
- Normal operation — clear sight glass confirms correct charge
- The sight glass location may be downstream of a restriction masking low charge (Correct answer)
- Refrigerant overcharge with moisture contamination
- Non-condensables causing pressure imbalance
Correct answer: The sight glass location may be downstream of a restriction masking low charge
A restriction such as a blocked filter-drier upstream of the sight glass can back up liquid and show a clear glass even when charge is insufficient.
RSES Troubleshooting 5
A rooftop packaged unit condenser fan motor fails in summer.
What will happen to system operation if ignored?