In what situation might you expect bubbles to appear in a sight glass?

Bubbles appearing in a sight glass typically indicate that the refrigerant level in the system is low. When the refrigerant level is deficient, the system cannot maintain proper pressure, and the liquid refrigerant may not fully condense before reaching the sight glass. This causes the incoming gas to mix with liquid refrigerant, resulting in bubbles being visible within the sight glass. A good refrigerant charge is necessary for the system to function efficiently and effectively, and the presence of bubbles serves as a clear visual indicator of this problem.

In contrast, observing bubbles would not suggest that the refrigerant level is normal or that the unit is overcharged, as a properly charged system would show a steady flow of liquid without bubbles. Additionally, operating in heating mode does not typically correlate with the presence of bubbles in the sight glass, as heating mode primarily relies on different refrigerant flow characteristics that are not indicative of low charge conditions.