Air Conditioning Repair System Diagnosis and Troubleshooting Questions and Answers — Questions and Answers
Question 1: A technician measures a very low superheat and an abnormally high subcooling on an air conditioning system that uses a thermostatic expansion valve (TXV). Which of the following is the most likely cause of these readings?
- Inefficient compressor
- Severe airflow restriction over the evaporator
- Refrigerant overcharge (Correct answer)
- Refrigerant undercharge
Correct answer: Refrigerant overcharge
High subcooling indicates that liquid refrigerant is backing up in the condenser, a classic sign of too much refrigerant. This excess refrigerant floods the evaporator, leading to very low superheat because the refrigerant cannot fully boil off before exiting the coil.
Question 2: A technician arrives at a service call to find the indoor evaporator coil is a solid block of ice. After properly and safely thawing the coil, which of the following should be the technician's FIRST diagnostic step?
- Check the refrigerant charge.
- Inspect the air filter and verify indoor airflow. (Correct answer)
- Measure the compressor amperage draw.
- Replace the thermostatic expansion valve.
Correct answer: Inspect the air filter and verify indoor airflow.
Insufficient airflow across the evaporator coil is the most common cause of freezing. The lack of air to transfer heat causes the coil's surface temperature to drop below 32°F (0°C). Checking the simplest and most common cause—airflow issues like a dirty filter—should always be the first step before investigating more complex refrigerant or component issues.
Question 3: A technician uses a multimeter to check the windings of a single-phase compressor motor. The readings are: Common to Run = 2 ohms, Common to Start = 6 ohms, and Run to Start = 8 ohms. A reading from any winding to the compressor casing shows infinity (OL). What do these readings indicate?
- A shorted winding.
- An open winding.
- The motor is shorted to ground.
- The motor windings are electrically sound. (Correct answer)
Correct answer: The motor windings are electrically sound.
For a single-phase motor, the resistance between the Run and Start terminals should equal the sum of the resistances from Common to Run and Common to Start (2 + 6 = 8). This confirms the windings have proper continuity. The infinity (OL) reading to the casing (ground) indicates there is no short to ground. Therefore, the windings are electrically intact.
Question 4: Which of the following is the most direct and primary indicator of non-condensables being present in a refrigeration system?
- Excessively high discharge (head) pressure. (Correct answer)
- Unusually low liquid line temperature.
- A completely frosted suction line.
- Compressor failing to start.
Correct answer: Excessively high discharge (head) pressure.
Non-condensable gases, such as air or nitrogen, do not change state in the condenser. They occupy valuable volume, reducing the surface area available for refrigerant to condense. This forces the compressor to work harder, resulting in head pressure that is significantly higher than normal for the given ambient conditions.
Question 5: While diagnosing a system, a technician observes low suction pressure, low superheat, and a frosted evaporator coil only near its inlet. Further inspection reveals an abnormally high subcooling reading. What is the most probable diagnosis?
- A failed indoor blower motor.
- A restriction in the liquid line filter-drier. (Correct answer)
- A severe refrigerant undercharge.
- An overcharged system.
Correct answer: A restriction in the liquid line filter-drier.
A restriction before the metering device (like in the filter-drier) causes refrigerant to back up in the condenser, leading to high subcooling. The restriction starves the evaporator of refrigerant, causing a large pressure drop across the blockage. This results in a very low suction pressure and causes the small amount of refrigerant that passes through to flash off and freeze immediately at the evaporator inlet, leading to low superheat readings.
Question 6: A technician uses a manometer to measure the total external static pressure (TESP) of an air handler and finds it to be 1.2 inches of water column (in. w.c.). The unit's data plate specifies a maximum TESP of 0.5 in. w.c. What does this high reading indicate?
- The blower motor is running too slowly.
- There is significant leakage in the supply ductwork.
- The return air filter is clean and the ductwork is oversized.
- There is excessive resistance to airflow in the duct system. (Correct answer)
Correct answer: There is excessive resistance to airflow in the duct system.
Total External Static Pressure (TESP) measures the resistance the blower fan works against to move air through the duct system. A reading that is significantly higher than the manufacturer's specification indicates excessive resistance, which can be caused by a dirty filter, clogged evaporator coil, undersized or poorly designed ductwork, or blocked registers.
Air Conditioning Repair System Diagnosis and Troubleshooting Questions and Answers
A technician measures a very low superheat and an abnormally high subcooling on an air conditioning system that uses a thermostatic expansion valve (TXV).
Which of the following is the most likely cause of these readings?