CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A technician measures suction line temperature at 55°F on a fixed-orifice system. The suction pressure gauge reads a saturation temperature of 40°F. What is the superheat, and what does it generally indicate about the charge if the target superheat is 10°F?
- A15°F superheat; system is overcharged
- B5°F superheat; system is undercharged
- C5°F superheat; system is overcharged
- D15°F superheat; system is undercharged
Show answer & explanationAnswer & explanation
Correct answer: D. 15°F superheat; system is undercharged
Superheat equals suction line temperature minus saturation temperature at suction pressure: 55°F − 40°F = 15°F. Since the measured superheat (15°F) exceeds the target (10°F), this indicates too little refrigerant is boiling off in the evaporator early, meaning the system is undercharged.
Why the other options are wrong
- A. The math is correct but overcharge would produce low superheat, not high.
- B. The subtraction is incorrect; 55−40=15, not 5.
- C. Both the math and the interpretation are incorrect.
Superheat Calculation
Superheat is the difference between the actual temperature of refrigerant vapor and its saturation temperature at the measured pressure, indicating how much liquid has fully vaporized.
- Superheat = actual suction line temp − saturation temp at suction pressure
- High superheat often indicates undercharge or restriction
- Low superheat often indicates overcharge
- Measured on fixed-orifice metering device systems for charging
Memory trick: High Superheat = Hungry system (needs more charge).