Air Conditioning Repair Installation and Evacuation Procedures 5 — Questions and Answers
Question 1: What is the purpose of the 'decay test' (also called a 'rise test') performed after reaching target vacuum?
- To verify system tightness by monitoring micron levels after pump isolation (Correct answer)
- To test the expansion valve at low pressure
- To measure refrigerant charge weight
- To check electrical continuity of the compressor
Correct answer: To verify system tightness by monitoring micron levels after pump isolation
After isolating the vacuum pump, micron levels are monitored for 15–30 minutes; a stable reading confirms system integrity, while a rise indicates leaks or moisture.
Question 2: When sizing the electrical disconnect for an outdoor condensing unit, the technician should reference:
- The unit's Maximum Overcurrent Protection (MOP) rating on the nameplate (Correct answer)
- The total BTU capacity divided by 1000
- The wire gauge of the existing service panel
- The compressor horsepower rating only
Correct answer: The unit's Maximum Overcurrent Protection (MOP) rating on the nameplate
The unit nameplate lists the MOP (Maximum Overcurrent Protection) value, which determines the correct fuse or breaker size for the disconnect.
Question 3: Refrigerant line sets should be supported at intervals not exceeding:
- Every 4–6 feet horizontally and every 8–10 feet vertically (Correct answer)
- Every 20 feet horizontally only
- Only at the unit connections — mid-span support is not required
- Every 12 inches to prevent vibration
Correct answer: Every 4–6 feet horizontally and every 8–10 feet vertically
Proper support prevents sagging, vibration fatigue, and potential kinks; horizontal runs need support every 4–6 feet and vertical runs every 8–10 feet.
Question 4: What problem occurs when refrigerant suction line velocities are too low in a vertical riser?
- Compressor oil fails to return and accumulates in the evaporator (Correct answer)
- The system overcharges automatically
- Suction pressure rises above normal
- The metering device becomes restricted
Correct answer: Compressor oil fails to return and accumulates in the evaporator
Oil circulates with refrigerant; insufficient velocity in vertical suction risers allows oil to drain back to the evaporator instead of returning to the compressor.
Question 5: A technician finds that the vacuum gauge reads 300 microns but then rises to 1,500 microns and stabilizes. This most likely indicates:
- Moisture is present in the system (outgassing) (Correct answer)
- A leak in the system
- The vacuum pump oil needs changing
- The gauge calibration is off
Correct answer: Moisture is present in the system (outgassing)
A vacuum that rises then stabilizes at a mid-range level (not continuing to rise) indicates moisture outgassing — the water is vaporizing and raising pressure until it is pulled out.
Question 6: According to NEC requirements, the minimum wire gauge for a typical 240V, 30-amp residential AC circuit is:
- 10 AWG copper (Correct answer)
- 14 AWG copper
- 12 AWG copper
- 8 AWG copper
Correct answer: 10 AWG copper
NEC requires 10 AWG copper conductors for a 30-amp, 240V circuit to safely carry the load without excessive voltage drop or heat buildup.
Question 7: When installing a heat pump in a northern climate, the outdoor unit base should be elevated above grade to:
- Prevent ice and snow from blocking the unit during defrost cycles (Correct answer)
- Improve refrigerant velocity in the lineset
- Satisfy local zoning setback requirements
- Reduce noise transmission to the ground
Correct answer: Prevent ice and snow from blocking the unit during defrost cycles
Elevating the outdoor unit on a raised pad or legs keeps it above typical snow accumulation levels, ensuring airflow and preventing ice blockage during defrost.
Air Conditioning Repair Installation and Evacuation Procedures 5
What is the purpose of the 'decay test' (also called a 'rise test') performed after reaching target vacuum?