CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A technician observes continuous bubbles moving through the liquid line sight glass of an operating R-410A split system. What does this most likely indicate?
- AAn overcharged condition with excess liquid refrigerant
- BExcessive subcooling in the liquid line
- CNormal operation expected on all TXV-equipped systems
- DLow refrigerant charge or a restriction causing flash gas to form in the liquid line
Show answer & explanationAnswer & explanation
Correct answer: D. Low refrigerant charge or a restriction causing flash gas to form in the liquid line
A fully liquid-filled line (adequately subcooled) should appear clear in the sight glass. Continuous bubbles indicate the liquid is partially flashing to vapor, typically due to low refrigerant charge or a restriction (e.g., clogged filter drier) causing a pressure drop before the metering device.
Why the other options are wrong
- A. Overcharge typically produces a clear sight glass with high subcooling, not bubbles.
- B. Excessive subcooling would produce a clear sight glass, not bubbles.
- C. Bubbles are not a normal steady-state condition on a properly charged system.
Sight Glass Bubbles
Bubbles in the liquid line sight glass indicate flash gas forming in the liquid line, typically caused by low refrigerant charge or a restriction.
- Clear sight glass = properly subcooled liquid
- Bubbles = flash gas from undercharge or restriction
- Some systems have no sight glass on zeotropic blends
Memory trick: Bubbles mean trouble — low charge or blockage.