Coil freeze occurs when evaporator temperature drops below 32 degrees Fahrenheit, causing moisture on the coil to freeze solid and block airflow. This condition develops from low refrigerant charge, restricted airflow, or dirty coils. A frozen evaporator coil prevents heat absorption and forces the system to shut down due to low-pressure protection.
Causes and Development
Refrigerant undercharge reduces pressure and superheat, allowing evaporator temperature to drop excessively. Airflow restrictions from dirty filters, closed dampers, or blocked ducts prevent warm return air from reaching the coil. Dirty evaporator coils reduce surface area available for heat transfer, requiring colder temperatures to achieve cooling capacity. Frozen coils typically develop within 30 minutes to 2 hours of operation.
Detection and Resolution
Visible ice buildup on indoor coil or visible frost on refrigerant lines indicates freezing. Technicians immediately stop system operation to prevent water damage and compressor liquid slugging. Recovery procedures include thawing the coil naturally over 2-4 hours with system off. After identifying and correcting the root cause—charge leak, airflow restriction, or coil cleaning—the system is evacuated and properly recharged before restarting.