What causes frost to form on a suction line?

Frost formation on a suction line typically occurs when the evaporator coil is not getting sufficient airflow, which can happen when a dirty filter restricts airflow. When the airflow to the evaporator coil is starved, it leads to decreased heat exchange, causing the temperature of the refrigerant in the suction line to drop significantly. This drop in temperature can result in moisture in the air condensing and freezing on the surface of the suction line.

In contrast, other factors like high refrigerant levels or excessive airflow do not directly cause frost to form in the same manner. A high refrigerant level would generally lead to other issues, such as inefficient cooling or potential flooding in the compressor. An overheated compressor typically indicates a different problem, such as lack of refrigerant or mechanical failures, and does not cause frost. Excessive airflow may actually help to prevent frost formation, as it tends to keep the evaporator coil warmer by promoting better heat transfer. Thus, a dirty filter leading to a starved evaporator is the scenario that correctly explains frost formation on the suction line.