CSLB C-20 HVAC ContractorCooling and RefrigerationHard
A technician measures compressor discharge line temperature at 260°F on an R-410A scroll compressor system where the condensing temperature (from head pressure) corresponds to 125°F saturation. What does this reading most likely indicate?
- ANormal operation expected for high-efficiency scroll compressors under any load
- BThe system is significantly overcharged with refrigerant
- CAn excessively high discharge temperature indicating a potential problem such as low refrigerant charge, restricted airflow, or worn compressor valves, risking oil breakdown
- DThe TXV is overfeeding the evaporator, causing compressor flooding
Show answer & explanationAnswer & explanation
Correct answer: C. An excessively high discharge temperature indicating a potential problem such as low refrigerant charge, restricted airflow, or worn compressor valves, risking oil breakdown
Manufacturers typically limit compressor discharge line temperatures to around 225°F for R-410A scroll compressors to prevent oil breakdown and carbon formation. A reading of 260°F, well above this limit, signals a fault condition such as low charge, poor airflow, or failing valves rather than normal operation.
Why the other options are wrong
- A. 260°F far exceeds safe manufacturer limits; this is not normal operation.
- B. Overcharge typically lowers discharge superheat and temperature, not raises it this high.
- D. Flooding from an overfeeding TXV would lower, not raise, discharge temperature.
Discharge Line Temperature Limit
Compressor discharge temperature should generally stay below about 225°F for R-410A scroll compressors; excessive temperatures indicate faults and risk oil breakdown.
- High discharge temp causes oil carbonization and acid formation
- Common causes: low charge, dirty condenser, failing valves
- Measured with a clamp-on thermocouple on the discharge line
Memory trick: Too hot at discharge cooks the oil.