HVAC HVAC Troubleshooting Techniques 4 — Questions and Answers
Question 1: A technician recovers refrigerant and finds the system has a deep vacuum after equalization. What does this indicate?
- The system was properly evacuated before service
- The system had a leak and lost most of its charge (Correct answer)
- The compressor has high discharge efficiency
- The filter-drier is plugged
Correct answer: The system had a leak and lost most of its charge
A system that reaches a deep vacuum upon gauge connection has lost refrigerant through a leak, leaving sub-atmospheric pressure inside.
Question 2: A zoned HVAC system heats one zone but fails to condition a second zone. The zone 2 damper motor is receiving 24V. What should the technician check next?
- Refrigerant charge in the main system
- Mechanical damper blade for binding or disconnected linkage (Correct answer)
- Zone 1 thermostat calibration
- Blower motor amperage
Correct answer: Mechanical damper blade for binding or disconnected linkage
If the actuator has power but the damper does not move, the mechanical damper blade itself may be physically stuck, corroded, or the linkage arm may be detached.
Question 3: A mini-split indoor unit drips water onto the floor during cooling operation. What is the FIRST thing to inspect?
- Refrigerant charge level
- Condensate drain line for blockage or improper slope (Correct answer)
- Blower wheel for ice accumulation
- Compressor discharge temperature
Correct answer: Condensate drain line for blockage or improper slope
Water dripping from a mini-split indoor unit almost always results from a blocked or improperly pitched condensate drain line that causes the drain pan to overflow.
Question 4: Subcooling reads 3°F on a fixed-orifice system operating in 80°F ambient. What condition does this suggest?
- Overcharged refrigerant
- Low refrigerant charge (Correct answer)
- Condenser coil is oversized
- Evaporator fan motor is running too fast
Correct answer: Low refrigerant charge
Very low subcooling on a fixed-orifice system indicates insufficient liquid refrigerant in the condenser, which is a sign of a low charge.
Question 5: A gas furnace pressure switch trips on every heating cycle after 10 minutes of operation. The inducer motor amperage is normal. What is the MOST likely cause?
- Blocked condensate drain causing water to back up into the pressure switch hose (Correct answer)
- Faulty gas valve solenoid
- High-limit switch opening on temperature rise
- Dirty flame sensor
Correct answer: Blocked condensate drain causing water to back up into the pressure switch hose
On high-efficiency condensing furnaces, condensate backing up into the pressure switch hose ports is a frequent cause of nuisance pressure switch trips after initial operation.
Question 6: An air handler blower runs on high speed only, regardless of the thermostat fan speed setting. What is the MOST likely cause?
- Failed ECM motor control module or incorrect dip-switch settings (Correct answer)
- Low refrigerant charge causing the system to run longer
- Undersized ductwork creating excessive static pressure
- Faulty high-pressure cutout switch
Correct answer: Failed ECM motor control module or incorrect dip-switch settings
An ECM blower that ignores speed commands usually has a failed control module or incorrect programming/dip-switch configuration that defaults to maximum speed.
Question 7: A technician performs a nitrogen pressure test on a new refrigerant line set and observes pressure dropping from 300 psig to 285 psig over 15 minutes. What should the technician do?
- Accept the reading as within tolerance and proceed to evacuation
- Locate and repair the leak before proceeding (Correct answer)
- Evacuate the line set immediately
- Add refrigerant to compensate for the pressure drop
Correct answer: Locate and repair the leak before proceeding
Any pressure drop during a nitrogen leak test indicates a leak that must be found and repaired before the system is evacuated and charged.
HVAC HVAC Troubleshooting Techniques 4
A technician recovers refrigerant and finds the system has a deep vacuum after equalization.
What does this indicate?