HVAC Troubleshooting 2 — Questions and Answers
Question 1: A heat pump is running in heating mode but the discharge air feels cool. What is the most likely cause?
- Reversing valve stuck in cooling position (Correct answer)
- Dirty air filter
- Low refrigerant charge
- Faulty thermostat anticipator
Correct answer: Reversing valve stuck in cooling position
A reversing valve stuck in the cooling position causes the heat pump to pump heat outdoors instead of indoors, resulting in cool discharge air during heating mode.
Question 2: An air conditioner short-cycles (turns on and off frequently). Which of the following is the LEAST likely cause?
- Oversized equipment
- Dirty evaporator coil (Correct answer)
- Low refrigerant charge
- Faulty contactor
Correct answer: Dirty evaporator coil
A dirty evaporator coil typically causes the unit to run longer or freeze up, not short-cycle; oversized equipment, low charge, and faulty contactors are common short-cycling causes.
Question 3: A technician measures suction pressure of 55 psig and discharge pressure of 175 psig on an R-22 system. What condition does this indicate?
- Normal operation
- Overcharge of refrigerant
- Low refrigerant charge (Correct answer)
- Restricted liquid line filter drier
Correct answer: Low refrigerant charge
Low suction and low discharge pressures on an R-22 system indicate a refrigerant undercharge, reducing the system's capacity.
Question 4: A gas furnace ignites but the burner shuts off after a few seconds. What component is most likely faulty?
- Draft inducer motor
- Flame sensor (flame rod) (Correct answer)
- Gas valve
- Pressure switch
Correct answer: Flame sensor (flame rod)
A dirty or failed flame sensor cannot verify the presence of a flame, causing the control board to shut off gas flow as a safety measure.
Question 5: A condensate drain line is clogged on a high-efficiency furnace. What symptom would you expect?
- Yellow burner flame
- Unit shuts down on pressure switch lockout (Correct answer)
- Tripped high-limit switch
- Loud banging at startup
Correct answer: Unit shuts down on pressure switch lockout
A clogged condensate drain causes water to back up and block the pressure switch port, causing the furnace to lockout on a pressure switch fault.
Question 6: Which tool is used to confirm that a compressor motor winding is not grounded to the compressor shell?
- Clamp-on ammeter
- Megohmmeter (megger) (Correct answer)
- Manifold gauge set
- Capacitor tester
Correct answer: Megohmmeter (megger)
A megohmmeter applies high voltage to measure insulation resistance, confirming whether a winding is grounded to the compressor shell.
Question 7: An evaporator coil repeatedly freezes. After verifying adequate airflow, the next most likely cause is:
- High outdoor ambient temperature
- Low refrigerant charge (Correct answer)
- Overcharged refrigerant
- High suction superheat
Correct answer: Low refrigerant charge
Low refrigerant charge reduces suction pressure and evaporator temperature below the freezing point of moisture in the air, causing coil icing.
HVAC Troubleshooting 2
A heat pump is running in heating mode but the discharge air feels cool.
What is the most likely cause?