How does a coil freeze in air conditioning system occurs when evaporator coil temperature drops below freezing point of water, causing moisture from air to form ice on coil fins and tubing. Normal evaporator coil operates above freezing around 40F, absorbs heat from indoor air. Coil freezes when airflow is insufficient to provide enough heat to refrigerant, or refrigerant pressure is too low reducing temperature. Process: warm indoor air passes over evaporator coil, refrigerant inside evaporates absorbing heat, if airflow restricted due to dirty filter, coil, or blower failure, less heat reaches coil, refrigerant does not fully evaporate and coil gets colder, temperature falls below 32F, moisture in air condenses and freezes. Also if low refrigerant charge, pressure drops, saturation temperature drops below freezing, same result. Once thin ice forms, it further blocks airflow and insulates coil, causing more freezing, ice spreads to suction line. Ice prevents heat exchange, cooling capacity drops, compressor may overheat. Defrost requires turning off cooling and running fan to melt ice, then fixing root cause like airflow or charge.
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