The answer to 'air filter freezing up' is that a frozen air filter is usually caused by a problem with the HVAC system, such as low refrigerant levels, a blocked airflow, or a malfunctioning blower fan. Symptoms include ice buildup on the filter, reduced airflow, and the system blowing cold air even when the thermostat is set to heat.
To fix a frozen air filter, you'll first need to identify and address the underlying issue. This may involve checking refrigerant levels, cleaning the blower, or replacing a faulty component. If you're not comfortable troubleshooting the system yourself, it's best to call a professional HVAC technician to diagnose and repair the problem.