Air Conditioning Repair Refrigeration System Repair 3 — Questions and Answers
Question 1: What does a high subcooling reading combined with high discharge pressure most likely indicate?
- Low refrigerant charge
- Non-condensables in the system (Correct answer)
- Faulty TXV
- Low condenser airflow
Correct answer: Non-condensables in the system
Non-condensables such as air in the system raise discharge pressure and subcooling because they occupy condenser space without condensing.
Question 2: When recovering refrigerant from a system, the technician must use equipment certified to which standard?
- UL 94
- SAE J2788 (Correct answer)
- ARI 700
- UL 1995
Correct answer: SAE J2788
SAE J2788 certification is required for refrigerant recovery/recycling equipment used on motor vehicle AC systems; EPA Section 608 requires similar certification for stationary systems.
Question 3: A tech notices the evaporator coil is freezing up only at the inlet. What is the most likely cause?
- Overcharge of refrigerant
- Low airflow across the evaporator
- Metering device feeding too much refrigerant (Correct answer)
- High suction superheat
Correct answer: Metering device feeding too much refrigerant
When the metering device feeds excessive refrigerant, it floods the evaporator inlet, causing ice formation at the entry point where liquid refrigerant accumulates.
Question 4: What is the minimum evacuation level (microns) typically required before recharging a residential air conditioning system?
- 10,000 microns
- 5,000 microns
- 500 microns (Correct answer)
- 100 microns
Correct answer: 500 microns
Industry standard requires pulling the system down to 500 microns or below to ensure adequate moisture and air removal before charging.
Question 5: A compressor has a burnt motor winding. After replacing it, what additional step is critical before restarting?
- Adding extra oil to the compressor
- Flushing the system to remove acid and debris (Correct answer)
- Increasing refrigerant charge by 10%
- Bypassing the crankcase heater
Correct answer: Flushing the system to remove acid and debris
A burnt motor leaves acid contamination that must be flushed out and a liquid-line suction filter-drier installed to prevent damage to the new compressor.
Question 6: Which type of oil is used in systems charged with R-410A refrigerant?
- Mineral oil
- Alkylbenzene oil
- Polyol ester (POE) oil (Correct answer)
- Polyalkylene glycol (PAG) oil
Correct answer: Polyol ester (POE) oil
POE oil is required in R-410A systems because it is miscible with HFC refrigerants and properly lubricates the compressor throughout the system.
Question 7: What does a negative superheat reading at the evaporator outlet indicate?
- Metering device is underfeeding
- System is low on charge
- Liquid refrigerant is leaving the evaporator (Correct answer)
- Condenser is oversized
Correct answer: Liquid refrigerant is leaving the evaporator
Negative superheat means the refrigerant temperature at the evaporator outlet is below saturation temperature, indicating liquid refrigerant is leaving the coil.
Air Conditioning Repair Refrigeration System Repair 3
What does a high subcooling reading combined with high discharge pressure most likely indicate?