CSLB C-20 HVAC ContractorCooling and RefrigerationHard
A technician is charging a new TXV split system by weight per the manufacturer's charging chart, then fine-tunes using subcooling. After charging, measured subcooling is 4°F, but the manufacturer specifies a target of 12°F. What should the technician do?
- AAdd refrigerant to the system, since low subcooling indicates undercharge
- BDo nothing, since subcooling is not affected by charge level on TXV systems
- CReplace the TXV, since subcooling should never differ from the target
- DRecover refrigerant to reduce the charge, since low subcooling indicates overcharge
Show answer & explanationAnswer & explanation
Correct answer: A. Add refrigerant to the system, since low subcooling indicates undercharge
On TXV systems, subcooling is the primary charging indicator. A subcooling reading below the target (4°F vs. 12°F target) indicates insufficient liquid refrigerant backed up in the condenser, meaning the system is undercharged, so refrigerant should be added until target subcooling is reached.
Why the other options are wrong
- B. Subcooling is directly affected by charge level and is the primary charging metric for TXV systems.
- C. A subcooling discrepancy from target is a charge issue, not necessarily a valve defect, and should be corrected by adjusting charge first.
- D. This is reversed—overcharge would cause subcooling to be higher than target, not lower.
TXV Charging by Subcooling
On TXV-equipped systems, technicians verify and adjust refrigerant charge primarily using subcooling measurements compared against manufacturer-specified targets.
- Low subcooling relative to target = undercharge, add refrigerant
- High subcooling relative to target = overcharge, recover refrigerant
- TXV systems use subcooling as primary charge check (not superheat)
- Always verify against manufacturer's specific target values
Memory trick: Low subcooling, low charge — add gas to balance the scale.