What does a frost build-up on the evaporator coils usually indicate?

A frost build-up on the evaporator coils typically indicates low refrigerant charge. This phenomenon occurs because a lower-than-normal refrigerant level can compromise the evaporator's ability to absorb heat effectively. When the refrigerant in the evaporator cannot adequately absorb heat from the air, it cools down excessively, leading to the formation of frost or ice on the coils.

In a properly functioning system, there should be a balance between the amount of refrigerant and the heat removal capacity of the evaporator. Low refrigerant levels can be caused by leaks in the system or inadequate servicing. As the refrigerant level drops, the evaporator cannot maintain the appropriate pressure and temperature required for efficient heat exchange, resulting in frost accumulation.

The other options do not accurately represent the causes of frost build-up on the coils. For example, a properly functioning unit would not experience frost formation as it should maintain the appropriate refrigerant charge and heat exchange. Excessive refrigerant flow could lead to poor operation, but it is more commonly associated with flooding rather than frost. Lastly, high ambient temperatures would typically cause the system to work harder but would not inherently cause frost on the evaporator coils when the unit is functioning correctly.