HVAC HVAC Troubleshooting Techniques 5 — Questions and Answers
Question 1: A rooftop unit supply air temperature is 58°F but the leaving water temperature from the chiller is correct at 44°F. What is the MOST likely cause of the warm supply air?
- Chilled water control valve is not opening fully (Correct answer)
- Refrigerant overcharge in the chiller
- Condenser water pump failure
- Blower wheel rotating backwards
Correct answer: Chilled water control valve is not opening fully
If chilled water supply temperature is correct but the coil is not cooling adequately, the control valve is restricting water flow through the coil, reducing heat transfer.
Question 2: A technician uses an electronic refrigerant detector and gets a strong signal near the compressor crankcase. What should the technician do to confirm the leak location?
- Immediately add refrigerant to bring pressure up for better detection
- Apply nitrogen pressure and use soap bubbles or UV dye to pinpoint the exact leak site (Correct answer)
- Replace the compressor without further testing
- Assume the detector is malfunctioning and check the condenser instead
Correct answer: Apply nitrogen pressure and use soap bubbles or UV dye to pinpoint the exact leak site
Electronic detectors identify general leak areas, but nitrogen pressure combined with soap solution or UV dye is needed to pinpoint the exact leak location before repair.
Question 3: A customer reports that their ductless heat pump's heating capacity drops significantly when outdoor temperature falls below 35°F. This is MOST likely caused by:
- A defective reversing valve sticking in cooling mode
- The normal thermodynamic reduction in heat pump heating capacity at low outdoor temperatures (Correct answer)
- A refrigerant overcharge condition
- A failed start capacitor on the outdoor fan motor
Correct answer: The normal thermodynamic reduction in heat pump heating capacity at low outdoor temperatures
Heat pump heating capacity naturally decreases as outdoor temperature drops because less heat energy is available to absorb from cold outdoor air—this is normal operation, not a fault.
Question 4: After replacing a compressor, a technician pulls a vacuum to 500 microns and the system holds for 15 minutes. The technician then charges the system and it trips the high-pressure switch. What was MOST likely missed during the compressor replacement?
- The filter-drier was not replaced (Correct answer)
- The system was not leak-tested with nitrogen before evacuation
- The reversing valve was not replaced
- The run capacitor was not checked
Correct answer: The filter-drier was not replaced
Replacing a failed compressor without replacing the filter-drier leaves acid and contaminants in the system, which can cause the new compressor to fail or cause restriction that raises head pressure.
Question 5: A building's VAV system delivers poor cooling to perimeter offices in the afternoon despite adequate static pressure at the AHU. What is the MOST likely cause?
- The chiller is undersized for the building load
- VAV box actuators in perimeter zones are failing to open due to solar heat gain overloading the zone controllers (Correct answer)
- The AHU economizer is locked open
- Supply air temperature is too high
Correct answer: VAV box actuators in perimeter zones are failing to open due to solar heat gain overloading the zone controllers
Afternoon solar gain dramatically increases perimeter zone loads, and if the VAV boxes cannot fully open or are undersized for peak solar load, those zones will overheat despite adequate system pressure.
Question 6: A walk-in cooler compressor short-cycles on the low-pressure cutout. The box temperature is above setpoint and the suction line is frosted to the compressor. What is the MOST likely cause?
- Refrigerant overcharge
- Evaporator coil heavily iced over due to a defrost failure (Correct answer)
- Condenser fan motor failure
- High ambient temperature in the equipment room
Correct answer: Evaporator coil heavily iced over due to a defrost failure
A failed defrost system allows ice to accumulate on the evaporator coil, blocking airflow and causing suction pressure to fall below the low-pressure cutout setting while the box warms up.
Question 7: A technician finds that the outdoor unit on a split system is louder than normal and vibrating excessively. Refrigerant pressures and temperatures are within normal range. What is the MOST likely cause?
- Low refrigerant charge affecting compressor noise
- Loose or failed outdoor fan blade, worn motor bearings, or broken compressor mounts (Correct answer)
- High subcooling caused by overcharge
- Faulty reversing valve chattering
Correct answer: Loose or failed outdoor fan blade, worn motor bearings, or broken compressor mounts
Excessive vibration and noise with normal refrigerant performance points to mechanical issues such as an unbalanced or cracked fan blade, worn motor bearings, or deteriorated compressor isolation mounts.
HVAC HVAC Troubleshooting Techniques 5
A rooftop unit supply air temperature is 58°F but the leaving water temperature from the chiller is correct at 44°F.
What is the MOST likely cause of the warm supply air?