HVAC Troubleshooting 5 — Questions and Answers
Question 1: An air conditioning system has normal suction pressure but very high discharge pressure. Outdoor ambient temperature is 85°F. What is the most probable cause?
- Low refrigerant charge
- Dirty or blocked condenser coil (Correct answer)
- Failed evaporator fan motor
- Low suction superheat
Correct answer: Dirty or blocked condenser coil
A dirty or blocked condenser coil restricts airflow and reduces heat rejection, causing discharge pressure to rise well above normal for the ambient temperature.
Question 2: A technician notices that a system's compressor trips on its internal overload protector repeatedly. Amp draw is below nameplate. What should be suspected?
- Electrical overload from undersized wiring
- Refrigerant overcharge flooding the compressor with liquid
- High ambient temperature around the compressor (Correct answer)
- Faulty start capacitor
Correct answer: High ambient temperature around the compressor
Even with normal amperage, excessive ambient temperature around the compressor can cause the thermal overload to trip as the compressor overheats.
Question 3: A ductless mini-split displays an error code and the outdoor unit produces a grinding noise. What is the most likely component causing the noise?
- Reversing valve solenoid
- Outdoor fan motor bearings (Correct answer)
- Compressor scroll plates
- Electronic expansion valve
Correct answer: Outdoor fan motor bearings
Worn or failing fan motor bearings produce a grinding noise in the outdoor unit and may trigger a fault code due to reduced or erratic fan speed.
Question 4: A technician finds that a boiler is not firing even though the thermostat is calling for heat. The system has 24V at the thermostat. What should be checked next?
- Low water cutoff device (Correct answer)
- Expansion tank pressure
- Flue pipe draft
- Circulator pump operation
Correct answer: Low water cutoff device
A low water cutoff (LWCO) shuts down the burner when water level or pressure is insufficient, and is a common safety-related cause of a boiler not firing.
Question 5: After startup in the spring, a split system produces adequate cooling but has a musty odor. What is the most appropriate corrective action?
- Add refrigerant to improve coil temperature
- Clean evaporator coil and condensate drain with appropriate cleaner and biocide (Correct answer)
- Replace the air filter
- Check for refrigerant leaks
Correct answer: Clean evaporator coil and condensate drain with appropriate cleaner and biocide
Musty odors from HVAC systems are caused by mold and bacterial growth on the wet evaporator coil and in the condensate drain, requiring cleaning and biocide treatment.
Question 6: A technician is troubleshooting a system where the compressor runs but the condenser fan does not. The fan capacitor tests good. What is the next component to check?
- Thermostat heat anticipator
- Condenser fan motor windings (Correct answer)
- Suction line filter drier
- High-pressure switch
Correct answer: Condenser fan motor windings
If the capacitor is good and voltage is present at the fan motor, failed motor windings (open winding) would prevent the fan from running.
Question 7: A VRF system outdoor unit operates but one of the indoor zones does not cool. What is the most likely cause specific to VRF systems?
- Failed compressor
- Faulty electronic expansion valve (EEV) at the non-cooling indoor unit (Correct answer)
- Refrigerant undercharge in outdoor unit
- Faulty reversing valve
Correct answer: Faulty electronic expansion valve (EEV) at the non-cooling indoor unit
In VRF systems, each indoor unit has its own EEV that modulates refrigerant flow; a failed EEV at a single zone prevents refrigerant from entering that unit while others continue to operate normally.
HVAC Troubleshooting 5
An air conditioning system has normal suction pressure but very high discharge pressure.
Outdoor ambient temperature is 85°F.
What is the most probable cause?