HVAC Troubleshooting 4 — Questions and Answers
Question 1: A technician notices that a system's high-side pressure is normal but the low-side pressure is higher than expected with low superheat. What is the most likely diagnosis?
- Restricted filter drier
- Overcharged refrigerant system
- Metering device stuck open or failed open (Correct answer)
- Undersized condensing unit
Correct answer: Metering device stuck open or failed open
A metering device stuck open floods the evaporator, raising suction pressure and lowering superheat significantly.
Question 2: A residential heat pump operates correctly in cooling but has poor performance in heating mode. What should be suspected?
- Dirty air filter
- Defrost control malfunction (Correct answer)
- Low refrigerant charge
- Faulty contactor
Correct answer: Defrost control malfunction
A faulty defrost control prevents the heat pump from clearing ice from the outdoor coil, drastically reducing heating capacity.
Question 3: During a service call, a technician measures 240V at a condenser but the unit does not start. The capacitor tests good. What should be checked next?
- Thermostat wiring
- Contactor contacts (Correct answer)
- Refrigerant charge
- Condenser fan blade
Correct answer: Contactor contacts
Burned or pitted contactor contacts may not pass current even when the contactor coil pulls in, preventing the compressor and fan from starting.
Question 4: An HVAC technician is troubleshooting a variable-speed air handler that is not communicating with the thermostat. What is the first step?
- Replace the control board
- Verify 24V power at the air handler control board (Correct answer)
- Replace the thermostat
- Check refrigerant charge
Correct answer: Verify 24V power at the air handler control board
Without 24V control voltage, the air handler's control board cannot operate or communicate, making power verification the essential first step.
Question 5: A gas furnace's pressure switch opens during operation. The flue pipe is clear and the inducer runs. What else should be checked?
- Gas valve pressure
- Condensate drain trap and tubing to the pressure switch (Correct answer)
- Burner flame sensor
- Heat exchanger integrity
Correct answer: Condensate drain trap and tubing to the pressure switch
A blocked condensate drain trap or kinked tubing to the pressure switch can cause a false negative-pressure reading, tripping the switch during operation.
Question 6: Subcooling is measured at 22°F on an R-410A system with a TXV. What condition does this indicate?
- Refrigerant undercharge
- Overcharge or restricted metering device (Correct answer)
- Normal operation
- Failed condenser fan
Correct answer: Overcharge or restricted metering device
Subcooling above 15°F on a TXV system typically indicates an overcharge or a restriction in the metering device causing liquid to back up in the condenser.
Question 7: A technician suspects a cracked heat exchanger in a gas furnace. Which test is the most reliable field method?
- Combustion analysis of CO levels (Correct answer)
- Manifold gas pressure measurement
- Static pressure drop across the coil
- Checking flue temperature with a thermometer
Correct answer: Combustion analysis of CO levels
Elevated carbon monoxide in the supply air or living space, verified with a combustion analyzer, is the most reliable indicator of a cracked heat exchanger allowing flue gases to enter the airstream.
HVAC Troubleshooting 4
A technician notices that a system's high-side pressure is normal but the low-side pressure is higher than expected with low superheat.
What is the most likely diagnosis?