The answer to 'why is my zone control system freezing up' is that a frozen zone control system is typically caused by low refrigerant levels, a problem with the thermostat, or an issue with the zone dampers. This can lead to the coil freezing up and restricting airflow through the system.
Some common symptoms include poor heating or cooling performance, ice buildup on the coils or vents, and the system cycling on and off frequently. To fix the problem, you'll need to identify and address the underlying cause, which may involve recharging the refrigerant, adjusting the thermostat, or repairing or replacing the zone dampers. In some cases, the issue may require the attention of a professional HVAC technician.