Air Conditioning Repair Installation and Service Procedures 4 — Questions and Answers
Question 1: A technician is performing a standing vacuum test and the micron gauge rises from 300 to 500 microns over 10 minutes after isolating the pump. What does this indicate?
- The system is ready to charge
- Moisture is present in the system (Correct answer)
- There is a refrigerant leak
- The vacuum pump oil is contaminated
Correct answer: Moisture is present in the system
A vacuum that rises and levels off (rather than continuing to rise to atmospheric) indicates moisture boiling off in the vacuum — the system needs continued evacuation.
Question 2: What is the recommended method for testing a newly brazed joint for leaks before evacuating and charging a system?
- Visual inspection only
- Pressurize with nitrogen and use electronic leak detector or soap bubbles (Correct answer)
- Pressurize with refrigerant to 150 psig
- Apply a vacuum and watch the gauge
Correct answer: Pressurize with nitrogen and use electronic leak detector or soap bubbles
Nitrogen pressure testing with an electronic detector or soap bubbles is safe and code-compliant; using refrigerant for leak testing wastes refrigerant and is prohibited.
Question 3: On a cooling system with a TXV, the technician measures 12°F subcooling at the condensing unit outlet and 22°F subcooling at the indoor unit inlet. What does this indicate?
- The system is properly charged
- There is a restriction in the liquid line (Correct answer)
- The system is undercharged
- The condenser fan motor is failing
Correct answer: There is a restriction in the liquid line
A significant subcooling rise through the liquid line indicates a restriction (such as a clogged filter drier or kinked line) that is causing a pressure and temperature drop.
Question 4: When pulling a vacuum on an R-410A system, the target micron level before charging should be:
- 5,000 microns
- 2,000 microns
- 500 microns (Correct answer)
- 100 microns or less
Correct answer: 500 microns
Industry best practice (ACCA, ASHRAE) targets 500 microns or below before charging; this level ensures adequate moisture and non-condensable gas removal.
Question 5: A residential split system is being installed in a garage. What special consideration applies to the condensing unit's electrical disconnect?
- No disconnect is needed if the breaker is within 50 feet
- A fused disconnect must be within sight of and readily accessible from the unit (Correct answer)
- A GFCI outlet can substitute for a disconnect
- The disconnect must be rated for outdoor use only if the garage is unheated
Correct answer: A fused disconnect must be within sight of and readily accessible from the unit
NEC 440.14 requires a disconnect within sight of and readily accessible from the condensing unit to allow safe service lockout.
Question 6: When installing refrigerant line sets, which practice is most important to prevent future service problems?
- Run the lines in a straight line only
- Document the line set route and leave accessible service ports (Correct answer)
- Wrap joints with duct tape for protection
- Use the smallest tubing diameter possible to reduce cost
Correct answer: Document the line set route and leave accessible service ports
Documenting the line route and ensuring accessible service ports allows future technicians to diagnose and repair the system without damaging walls or ceilings.
Question 7: A new heat pump installation requires the thermostat wire to run 120 feet from the air handler to the thermostat. What adjustment may be necessary?
- Use 18-gauge wire instead of 20-gauge to reduce voltage drop (Correct answer)
- Install a thermostat signal booster
- Reduce the number of thermostat functions
- Use shielded cable to prevent interference
Correct answer: Use 18-gauge wire instead of 20-gauge to reduce voltage drop
Long thermostat wire runs can experience voltage drop that prevents controls from operating reliably; heavier 18-gauge wire reduces resistance over the longer distance.
Air Conditioning Repair Installation and Service Procedures 4
A technician is performing a standing vacuum test and the micron gauge rises from 300 to 500 microns over 10 minutes after isolating the pump.
What does this indicate?