The answer to 'why is my evaporator coil freezing up' is that it is likely due to low airflow over the coil, a refrigerant leak, or a problem with the thermostat or fan. When the evaporator coil freezes up, it can restrict airflow and reduce the cooling capacity of your HVAC system.
Some common causes of a frozen evaporator coil include a dirty air filter, a blocked return vent, a malfunctioning blower fan, or a refrigerant leak. If the coil is frozen, you'll typically notice reduced cooling, higher energy bills, and potentially water leaking from the system. In most cases, it's best to call a professional HVAC technician to diagnose and repair the issue, as they have the tools and expertise to properly troubleshoot and fix the problem.