The answer to 'why is my zone control system blowing cold air' is that there is likely an issue with the thermostat, dampers, or control board in your HVAC zone control system. This could be caused by a malfunction, improper programming, or a problem with the airflow in that zone.
Some common symptoms include uneven temperatures, rooms that are too hot or too cold, and the system blowing cold air when it should be heating. The solution may involve adjusting the thermostat settings, checking the dampers and airflow, or troubleshooting the control board and wiring. In some cases, you may need to call an HVAC technician to diagnose and repair the issue.