The answer to 'why is my zone control system freezing up' is that it is likely due to a problem with the system's refrigerant charge or airflow. A common cause is low refrigerant, which can lead to the evaporator coil freezing up and restricting airflow. Another potential issue is a blockage or restriction in the airflow, such as a dirty air filter or a problem with the blower fan.
If your zone control system is freezing up, you may notice reduced airflow, ice buildup on the coils, or the system cycling on and off frequently. In some cases, the system may even shut down completely to prevent further damage.