CRST Troubleshooting & Repair Techniques 5 — Questions and Answers
Question 1: A heat pump in heating mode has frost building up on the outdoor coil when outdoor temperature is above 40°F. What is the MOST likely cause?
- Normal defrost cycle operation
- Low refrigerant charge reducing evaporating temperature (Correct answer)
- High indoor fan speed
- Oversized reversing valve
Correct answer: Low refrigerant charge reducing evaporating temperature
A low refrigerant charge drops the evaporating temperature below the frost point even in mild weather, causing premature frost accumulation on the outdoor coil.
Question 2: A technician uses electronic leak detection and finds a leak at the evaporator coil. What is the NEXT step before brazing the repair?
- Pressurize with nitrogen to 400 PSIG
- Recover the refrigerant from the system (Correct answer)
- Add leak sealant to the charge
- Weld the coil while the system is under pressure
Correct answer: Recover the refrigerant from the system
Before any brazing or open-flame work, refrigerant must be completely recovered to prevent combustion of refrigerant decomposition products and to comply with EPA regulations.
Question 3: What is the MOST effective way to remove non-condensables from a refrigeration system?
- Run the system at high load to purge them through the condenser
- Recover refrigerant, evacuate the system, and recharge with virgin refrigerant (Correct answer)
- Add refrigerant to dilute the non-condensables
- Install a purge valve on the high side
Correct answer: Recover refrigerant, evacuate the system, and recharge with virgin refrigerant
Non-condensables cannot be separated in the field; the only correct method is full recovery, deep evacuation to remove air and moisture, and recharge with new refrigerant.
Question 4: A compressor with a cracked discharge valve reed will MOST likely exhibit:
- High discharge pressure and normal suction pressure
- Low compression ratio and elevated suction pressure (Correct answer)
- Excessive oil consumption only
- Normal pressures but high amperage
Correct answer: Low compression ratio and elevated suction pressure
A cracked discharge valve allows high-pressure gas to re-expand into the cylinder, reducing pumping efficiency and causing elevated suction pressure and low compression ratio.
Question 5: When troubleshooting a system that repeatedly trips on high-pressure cutout, the technician should inspect the:
- Suction strainer and accumulator
- Condenser airflow, refrigerant charge, and non-condensables (Correct answer)
- Low-pressure switch calibration
- Crankcase heater and oil level
Correct answer: Condenser airflow, refrigerant charge, and non-condensables
High head pressure is caused by poor condenser heat rejection from blocked airflow, refrigerant overcharge, or non-condensables — all three must be checked.
Question 6: A technician notices the suction pressure is pulsing rhythmically on the gauge. This is MOST often caused by:
- TXV hunting due to improper superheat setting or bulb position (Correct answer)
- A defective low-pressure switch
- Refrigerant overcharge
- Condenser fan cycling
Correct answer: TXV hunting due to improper superheat setting or bulb position
TXV hunting occurs when the valve over-corrects repeatedly, causing cyclic swings in suction pressure that appear as gauge pulsation.
Question 7: After repairing a refrigerant leak and recharging a system, the technician should verify proper charge by checking:
- Only the sight glass clarity
- Superheat or subcooling depending on the metering device type (Correct answer)
- Discharge temperature only
- Compressor amperage only
Correct answer: Superheat or subcooling depending on the metering device type
Superheat is used to verify charge on fixed orifice systems, while subcooling is used on TXV systems; both methods confirm accurate refrigerant quantity in the circuit.
CRST Troubleshooting & Repair Techniques 5
A heat pump in heating mode has frost building up on the outdoor coil when outdoor temperature is above 40°F.
What is the MOST likely cause?