NCT HVAC System Components & Diagnostics 2 — Questions and Answers
Question 1: A technician measures superheat at the evaporator outlet and finds it is only 2°F. What condition does this indicate?
- Flooded evaporator with liquid refrigerant carry-over risk (Correct answer)
- Normal operating condition for a TXV system
- Overcharged system with excessive subcooling
- Clogged filter-drier restricting refrigerant flow
Correct answer: Flooded evaporator with liquid refrigerant carry-over risk
A superheat of 2°F at the evaporator outlet indicates a flooded coil, risking liquid refrigerant slugging the compressor.
Question 2: Which type of heat exchanger is most commonly used as a condenser in air-cooled commercial HVAC systems?
- Plate-and-frame heat exchanger
- Shell-and-tube heat exchanger
- Fin-and-tube coil with axial fans (Correct answer)
- Double-pipe counterflow exchanger
Correct answer: Fin-and-tube coil with axial fans
Air-cooled condensers in commercial HVAC systems typically use fin-and-tube coils with axial fans to reject heat to the outdoor air.
Question 3: What is the primary purpose of a crankcase heater on a refrigerant compressor?
- To preheat refrigerant vapor before compression
- To prevent refrigerant migration and liquid slugging at startup (Correct answer)
- To maintain oil viscosity during high-load operation
- To reduce compressor discharge temperature
Correct answer: To prevent refrigerant migration and liquid slugging at startup
Crankcase heaters prevent refrigerant from migrating into the oil during the off cycle, which would cause oil foaming and slugging at startup.
Question 4: A variable air volume (VAV) box fails to modulate airflow. Which component should the technician inspect first?
- Upstream duct insulation integrity
- Damper actuator and its control signal (Correct answer)
- Supply fan motor amperage draw
- Duct static pressure sensor calibration
Correct answer: Damper actuator and its control signal
The damper actuator is the primary modulating component of a VAV box, and a failed actuator or missing control signal is the most common cause of no modulation.
Question 5: During commissioning of a chilled water system, the technician notices the entering and leaving chilled water temperature difference (ΔT) is only 4°F instead of the design 10°F. What is the most likely cause?
- Chiller is undersized for the load
- Chilled water flow rate is too high (Correct answer)
- Chiller condenser is fouled
- Refrigerant charge is insufficient
Correct answer: Chilled water flow rate is too high
A low ΔT across the chiller evaporator typically indicates excessive chilled water flow rate, a common symptom of low ΔT syndrome.
Question 6: Which refrigerant circuit component prevents high-pressure liquid from flashing to vapor before reaching the metering device?
- Suction accumulator
- Liquid line filter-drier
- Liquid line solenoid valve
- Subcooler or liquid-to-suction heat exchanger (Correct answer)
Correct answer: Subcooler or liquid-to-suction heat exchanger
A subcooler or liquid-to-suction heat exchanger adds subcooling to the liquid refrigerant, preventing flash gas formation before the metering device.
Question 7: A rooftop unit's economizer damper remains fully open on a hot, humid day. Which component is most likely malfunctioning?
- Supply air temperature sensor
- Return air CO₂ sensor
- Enthalpy sensor or economizer controller (Correct answer)
- Mixed air temperature sensor
Correct answer: Enthalpy sensor or economizer controller
The enthalpy sensor or economizer controller determines whether outdoor conditions are suitable for free cooling; a faulty sensor can cause the damper to stay open inappropriately.
NCT HVAC System Components & Diagnostics 2
A technician measures superheat at the evaporator outlet and finds it is only 2°F.
What condition does this indicate?