CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A technician checks a residential AC system and records: liquid line saturation temperature 108°F, liquid line temperature 95°F, superheat 2°F, and head pressure higher than normal for the outdoor ambient temperature. The manufacturer's target subcooling is 10°F. What is the most likely cause of these readings?
- AThe system is undercharged
- BThe evaporator coil is dirty and restricting airflow
- CThe system is overcharged with refrigerant
- DThe metering device is stuck fully closed
Show answer & explanationAnswer & explanation
Correct answer: C. The system is overcharged with refrigerant
Subcooling = 108°F - 95°F = 13°F, which is above the 10°F target. Combined with abnormally low superheat (2°F) and elevated head pressure, this pattern is the classic signature of an overcharged system, since excess liquid refrigerant floods the condenser and raises subcooling and head pressure while starving the evaporator of superheat.
Why the other options are wrong
- A. Undercharge produces low subcooling and high superheat, the opposite of these readings.
- B. A dirty evaporator coil typically causes low suction pressure and airflow-related symptoms, not this subcooling pattern.
- D. A fully closed metering device would starve the evaporator, causing very high superheat and low suction pressure, not low superheat.
Overcharge Diagnostic Pattern
An overcharged system shows subcooling above target, superheat below target, and elevated head pressure due to excess liquid refrigerant backing up into the condenser.
- Subcooling = saturation temp − liquid line temp
- Overcharge raises subcooling and head pressure
- Overcharge lowers superheat toward zero
Memory trick: High sub, low super, high head — too much refrigerant instead.