Air Conditioning Repair Service 3 — Questions and Answers
Question 1: When using an electronic leak detector on an air conditioning system, the probe should be moved:
- Slowly along the bottom of fittings and connections (Correct answer)
- Rapidly across the entire unit
- Only along the top of refrigerant lines
- Directly over the Schrader valves only
Correct answer: Slowly along the bottom of fittings and connections
Refrigerant vapor is heavier than air and settles downward, so moving the probe slowly along the bottom of fittings ensures detection of leaking vapors.
Question 2: A technician replaces a contactor on a condensing unit. After the repair, the compressor hums but does not start. What is the most likely cause?
- Failed run capacitor (Correct answer)
- Open high-pressure switch
- Defective thermostat
- Clogged filter drier
Correct answer: Failed run capacitor
A humming compressor that won't start typically indicates a failed run or start capacitor that is not providing the necessary starting torque.
Question 3: What is the correct procedure when adding refrigerant to a system equipped with a TXV metering device?
- Charge by weight or target superheat at the evaporator outlet (Correct answer)
- Charge by subcooling only at the condenser
- Add refrigerant until sight glass clears
- Charge to manufacturer's listed operating pressure only
Correct answer: Charge by weight or target superheat at the evaporator outlet
TXV systems are best charged by weight per manufacturer specs or by targeting correct superheat, as sight glass clearing is not a reliable method for TXV systems.
Question 4: Which component's failure would most likely cause high superheat AND high subcooling simultaneously?
- Restricted liquid line or filter drier (Correct answer)
- Overcharged refrigerant
- Failed condenser fan motor
- Open suction service valve
Correct answer: Restricted liquid line or filter drier
A restriction in the liquid line or a clogged filter drier causes refrigerant to back up (high subcooling) before the restriction and starve the evaporator (high superheat) after it.
Question 5: What does a nitrogen purge during brazing accomplish?
- Prevents oxidation and carbon scale from forming inside the copper tubing (Correct answer)
- Increases the melting temperature of the solder
- Removes moisture from the atmosphere
- Pressurizes the system to check for leaks
Correct answer: Prevents oxidation and carbon scale from forming inside the copper tubing
Flowing dry nitrogen through the tubing during brazing displaces oxygen, preventing the formation of copper oxides (scale) that can contaminate the system.
Question 6: A split-system air conditioner has a normal suction pressure but an abnormally high discharge pressure. The MOST likely cause is:
- Dirty or blocked condenser coil (Correct answer)
- Low refrigerant charge
- Restricted filter drier
- Faulty TXV
Correct answer: Dirty or blocked condenser coil
A dirty condenser coil reduces heat rejection, causing refrigerant to remain at a higher temperature and pressure on the high side.
Question 7: When is it acceptable to mix different types of refrigerant in a single system?
- Never — mixing refrigerants is not permitted (Correct answer)
- Only when both are HFCs
- When the pressures are within 5 psig of each other
- Only if approved by the EPA in writing
Correct answer: Never — mixing refrigerants is not permitted
Mixing different refrigerants creates an unidentified blend that cannot be safely recovered, recycled, or identified, and is prohibited by EPA regulations.
Air Conditioning Repair Service 3
When using an electronic leak detector on an air conditioning system, the probe should be moved: