Freeze up occurs when ice accumulates on the evaporator coil, restricting airflow and severely reducing cooling capacity or causing system shutdown. This condition typically develops within hours and indicates low refrigerant, restricted airflow, or improper metering device operation. Freeze up represents an emergency requiring immediate diagnosis and repair to prevent compressor damage.
Causes and Indicators
Primary causes include refrigerant undercharge, dirty filters or blocked ducts restricting airflow below 300-400 CFM per ton, failed thermostatic expansion valve (TXV), and low ambient conditions below 60°F. Symptoms include frost visible on suction line, very low suction pressure below 20 psi, low superheat below 5°F, and ice on outdoor coil or indoor evaporator.
Emergency Response
Turn the system to fan-only mode to melt ice and prevent compressor liquid slugging. Do not operate in cooling mode until ice clears and root cause is corrected. Check filter cleanliness, verify return air temperature above 60°F, test refrigerant charge, and inspect metering devices. Continued operation risks $500-2000 compressor replacement.