NCT HVAC System Components & Diagnostics 3 — Questions and Answers
Question 1: What does a technician use a psychrometric chart to determine when troubleshooting an air handling unit?
- Refrigerant pressure-enthalpy relationships
- Air properties including dry-bulb, wet-bulb, humidity ratio, and enthalpy (Correct answer)
- Fan motor efficiency at various speeds
- Duct system static pressure loss
Correct answer: Air properties including dry-bulb, wet-bulb, humidity ratio, and enthalpy
A psychrometric chart graphically represents air properties such as temperature, moisture content, enthalpy, and relative humidity, essential for AHU troubleshooting.
Question 2: A scroll compressor is producing a loud rattling noise at startup. What is the most likely cause?
- Worn scroll tip seals causing refrigerant bypass
- Liquid refrigerant slugging the compressor at startup (Correct answer)
- Loose refrigerant piping vibrating against the cabinet
- Overloaded motor drawing excessive amperage
Correct answer: Liquid refrigerant slugging the compressor at startup
Liquid refrigerant or oil slugging is the most common cause of rattling or knocking in scroll compressors at startup, often due to refrigerant migration during the off cycle.
Question 3: In a split system with R-410A, the technician measures 420 psig on the high side at 95°F outdoor ambient. What does this indicate?
- Normal operating condition for R-410A at this ambient
- Non-condensable gases present in the system (Correct answer)
- Refrigerant undercharge reducing head pressure
- Restricted metering device reducing flow
Correct answer: Non-condensable gases present in the system
R-410A saturated condensing pressure at 95°F ambient is approximately 380-400 psig; elevated pressure above this range suggests non-condensable gases such as air or nitrogen.
Question 4: Which type of metering device maintains a constant evaporator pressure regardless of load changes?
- Thermostatic expansion valve (TXV)
- Automatic expansion valve (AXV) (Correct answer)
- Electronic expansion valve (EEV)
- Fixed orifice restrictor
Correct answer: Automatic expansion valve (AXV)
The automatic expansion valve (AXV) is designed to maintain constant evaporator pressure by modulating refrigerant flow in response to pressure changes.
Question 5: A cooling tower is experiencing white mineral deposits on the fill media. What is the corrective maintenance action?
- Increase blowdown rate and treat water to control scale formation (Correct answer)
- Replace the fill media immediately to restore airflow
- Increase fan speed to compensate for reduced heat transfer
- Add algaecide to the basin to prevent biological growth
Correct answer: Increase blowdown rate and treat water to control scale formation
Mineral scale on cooling tower fill is controlled by increasing blowdown to lower cycles of concentration and using scale inhibitors in the water treatment program.
Question 6: A technician is sizing ductwork using the equal friction method. What is the primary design parameter maintained constant throughout the duct system?
- Air velocity in feet per minute
- Friction loss per 100 feet of duct (Correct answer)
- Static pressure at each supply outlet
- Total CFM delivered to each zone
Correct answer: Friction loss per 100 feet of duct
The equal friction method sizes all duct sections to produce the same friction loss per 100 feet of equivalent length, simplifying design and balancing.
Question 7: What parameter does a manifold gauge set's compound (low-side) gauge measure that a standard pressure gauge cannot?
- Superheat temperature at the evaporator
- Both pressure and vacuum (inches of mercury) (Correct answer)
- Refrigerant moisture content
- Saturated condensing temperature
Correct answer: Both pressure and vacuum (inches of mercury)
A compound gauge reads both positive pressure and vacuum below atmospheric pressure (in inches of mercury), essential for system evacuation verification.
NCT HVAC System Components & Diagnostics 3
What does a technician use a psychrometric chart to determine when troubleshooting an air handling unit?