CSLB C-20 HVAC ContractorCooling and RefrigerationHard
A technician arrives at a service call where a system exhibits low suction pressure, high superheat, low subcooling, and reduced cooling capacity. Which condition most likely explains all of these symptoms together?
- ALow refrigerant charge due to a leak
- BRefrigerant overcharge
- CA failed TXV stuck in the fully open position
- DA dirty condenser coil restricting airflow
Show answer & explanationAnswer & explanation
Correct answer: A. Low refrigerant charge due to a leak
A refrigerant leak causing low charge produces low suction pressure (less refrigerant mass flow), high superheat (early evaporation in the coil), low subcooling (insufficient liquid backup in the condenser), and reduced cooling capacity—all symptoms matching a low-charge condition rather than airflow or valve issues.
Why the other options are wrong
- B. Overcharge typically produces high subcooling and possibly high suction pressure, opposite of these symptoms.
- C. A TXV stuck fully open would cause low superheat/flooding, not high superheat.
- D. A dirty condenser coil typically raises head pressure and subcooling rather than lowering them.
Low Charge Diagnostic Pattern
A system low on refrigerant charge typically shows low suction pressure, high superheat, low subcooling, and reduced capacity simultaneously.
- Low suction pressure due to reduced refrigerant mass flow
- High superheat from early vapor formation in evaporator
- Low subcooling from insufficient liquid refrigerant in condenser
- Common root cause is a refrigerant leak
Memory trick: Low charge = low everything except superheat, which climbs high.