If foam is seen in the sight glass, what does it indicate?

When foam is observed in the sight glass, it indicates that the refrigerant charge in the system is low. A low refrigerant charge can lead to insufficient refrigerant flow through the system, which causes the presence of foam instead of a clear liquid refrigerant in the sight glass. This foam is a result of the refrigerant not being fully condensed, which typically means that the evaporator is not getting enough refrigerant to absorb heat effectively, leading to inefficient cooling.

In a properly functioning system, the refrigerant should appear as a clear liquid in the sight glass when the system is operating correctly with the appropriate charge. Foam suggests a disruption in the necessary liquid refrigerant flow, reinforcing that maintaining the correct refrigerant levels is essential for optimal system performance.