HVAC HVAC Troubleshooting Techniques 3 — Questions and Answers
Question 1: A technician finds oil staining around a brazed refrigerant joint on a rooftop unit. What should be done before adding refrigerant?
- Add refrigerant and recheck pressures
- Repair the leak, pressure-test, evacuate, then recharge (Correct answer)
- Apply sealant to the joint and recharge
- Run the system to verify the leak seals itself
Correct answer: Repair the leak, pressure-test, evacuate, then recharge
EPA regulations require technicians to repair detectable leaks before recharging, followed by a pressure test and deep evacuation to ensure system integrity.
Question 2: An evaporative cooler delivers warm air despite the pump running. What is the MOST likely cause?
- Blower belt is broken
- Pads are mineral-scaled and restricting water distribution (Correct answer)
- Refrigerant charge is low
- Thermostat is miscalibrated
Correct answer: Pads are mineral-scaled and restricting water distribution
Heavily scaled or clogged evaporative pads prevent adequate water saturation, eliminating the evaporative cooling effect even when the pump operates.
Question 3: A geothermal heat pump shows high entering water temperature in cooling mode. What issue does this suggest?
- Undersized circulating pump or flow restriction in the ground loop (Correct answer)
- Low refrigerant charge
- Dirty air filter
- Faulty reversing valve
Correct answer: Undersized circulating pump or flow restriction in the ground loop
High entering water temperature in cooling mode indicates the ground loop is not rejecting heat properly, usually due to inadequate flow from a weak pump or blockage.
Question 4: A rooftop unit's economizer damper stays open even when outdoor air enthalpy is high. What is the likely fault?
- Supply fan motor overload
- Faulty enthalpy sensor or economizer controller (Correct answer)
- Low refrigerant charge
- Dirty evaporator coil
Correct answer: Faulty enthalpy sensor or economizer controller
An economizer that fails to close in high-enthalpy conditions points to a defective enthalpy sensor or a failed economizer control module that cannot signal the damper actuator.
Question 5: During a combustion analysis on a gas furnace, CO in the flue gas measures 600 ppm (air-free). What action is required?
- Note the reading and recheck next season
- Immediately shut down the unit and tag it out pending heat exchanger inspection (Correct answer)
- Increase gas pressure to improve combustion efficiency
- Clean the burners and retest
Correct answer: Immediately shut down the unit and tag it out pending heat exchanger inspection
CO readings above 400 ppm air-free in flue gas indicate incomplete combustion likely caused by a cracked heat exchanger, which poses an immediate life-safety hazard.
Question 6: A packaged unit compressor makes a grinding noise on startup but runs quietly once at speed. What is the MOST likely cause?
- Liquid slugging from refrigerant flood-back
- Loose refrigerant line vibrating against sheet metal
- Failed start capacitor causing slow motor start (Correct answer)
- Overcharged refrigerant system
Correct answer: Failed start capacitor causing slow motor start
A grinding or growling sound only during startup that resolves at speed is a classic sign of a weak start capacitor allowing the compressor motor to start slowly under high load.
Question 7: A technician measures static pressure drop across a filter at 0.65 in. w.c. on a system rated for 0.20 in. w.c. maximum. What is the expected outcome?
- Increased airflow velocity across the coil
- Reduced airflow leading to low evaporator temperatures and potential coil icing (Correct answer)
- Higher compressor discharge pressure
- Improved indoor air quality without performance impact
Correct answer: Reduced airflow leading to low evaporator temperatures and potential coil icing
Excessive filter restriction reduces airflow, causing the evaporator coil to drop below freezing temperature and eventually ice over, further compounding the restriction.
HVAC HVAC Troubleshooting Techniques 3
A technician finds oil staining around a brazed refrigerant joint on a rooftop unit.
What should be done before adding refrigerant?