Bubbles in the sight glass could indicate which condition in a system using a thermostatic expansion valve?

• A. Overcharged system
• B. Undercharge
• C. Low condenser pressure
• D. TEV with an excessive superheat setting

Answer: (D)

Bubbles in the sight glass show that liquid is flashing to vapor as it flows, which often points to insufficient liquid flow to the evaporator. In a system with a thermostatic expansion valve, the valve modulates the liquid refrigerant to maintain the desired evaporator superheat. If the superheat setting is too high, the valve stays more closed and delivers less liquid than the system needs. That restricted flow causes a pressure drop and the liquid can flash into vapor, appearing as bubbles in the sight glass. So this symptom is best explained by the expansion valve being set for excessive superheat. If needed, verify the target superheat, check the valve operation, and adjust toward the manufacturer's specification while ensuring proper charge and heat-load conditions.