RSES Troubleshooting 3 — Questions and Answers
Question 1: A reciprocating compressor has low discharge pressure and low suction pressure, but the compressor runs continuously. What is the most likely fault?
- Refrigerant overcharge
- Worn or leaking compressor valves (Correct answer)
- Plugged liquid line filter-drier
- Faulty condenser fan motor
Correct answer: Worn or leaking compressor valves
Worn or leaking compressor valves reduce pumping efficiency, resulting in low suction and discharge pressures with continuous runtime.
Question 2: During a service call, a technician finds the evaporator fan running but no airflow over the coil. What is the most likely cause?
- Low refrigerant charge
- Evaporator coil completely iced over (Correct answer)
- High suction superheat
- Overcharged system
Correct answer: Evaporator coil completely iced over
A fully iced evaporator coil blocks airflow even when the fan motor is running and spinning.
Question 3: A technician notices the suction line is frosting all the way back to the compressor. What condition does this indicate?
- High superheat at the evaporator outlet
- Flooded evaporator or excessive refrigerant flow (Correct answer)
- Restricted liquid line drier
- Low ambient temperature
Correct answer: Flooded evaporator or excessive refrigerant flow
Frost extending to the compressor indicates liquid refrigerant is present in the suction line, meaning the evaporator is flooded.
Question 4: Which instrument is used to measure superheat at the evaporator outlet?
- A megohmmeter
- A clamp-on ammeter
- A thermometer and manifold gauge set (Correct answer)
- A leak detector
Correct answer: A thermometer and manifold gauge set
Superheat is calculated by comparing the actual suction line temperature (thermometer) to the saturation temperature at suction pressure (manifold gauge).
Question 5: A system has normal suction pressure but abnormally high discharge pressure. The condenser coils are clean. What should be suspected?
- Non-condensable gases in the system (Correct answer)
- Refrigerant undercharge
- Faulty expansion valve stuck open
- Low condenser airflow due to fan blade slip
Correct answer: Non-condensable gases in the system
Non-condensable gases raise discharge pressure disproportionately because they do not condense and occupy condenser surface area.
Question 6: What is the correct procedure to verify a restricted filter-drier in a liquid line?
- Measure temperature drop across the drier and check for pressure drop (Correct answer)
- Check compressor amperage draw
- Measure suction superheat only
- Bypass the drier and measure subcooling
Correct answer: Measure temperature drop across the drier and check for pressure drop
A temperature drop (and pressure drop if gauges are installed) across the drier indicates a restriction causing flash gas before the expansion device.
Question 7: A heat pump in heating mode has low suction pressure but high discharge pressure. What is the most likely diagnosis?
- Defective reversing valve causing partial bypass (Correct answer)
- Refrigerant overcharge
- Dirty indoor coil acting as condenser
- Excessive load on the heat pump
Correct answer: Defective reversing valve causing partial bypass
A leaking reversing valve allows high-pressure discharge gas to mix with low-pressure suction gas, causing abnormal pressures in heating mode.
RSES Troubleshooting 3
A reciprocating compressor has low discharge pressure and low suction pressure, but the compressor runs continuously.
What is the most likely fault?