Which condition could indicate that the suction line is below the dew point?

Frost on the suction line is indicative of the suction line operating below the dew point. When the temperature of the suction line falls below the dew point temperature of the surrounding air, moisture in the air can condense and freeze on the surface of the line, forming frost. This occurs because as refrigerant gas within the suction line absorbs heat from the warmer air, the temperature of the line drops, and if it drops sufficiently low, condensation occurs, leading to frost formation.

In the context of HVAC systems, maintaining proper temperature within various components is crucial to ensuring efficiency and preventing issues such as icing or improper operation. Frost on the evaporator coil may suggest other operational issues, and while weather changes and high humidity can affect system performance, they do not directly indicate that the suction line itself is below the dew point. Therefore, the presence of frost specifically on the suction line is a clear sign that it has dipped below this critical temperature threshold.