The answer to 'why is my zone control system blowing cold air' is that there could be several common causes, such as a problem with the thermostat, a stuck or malfunctioning zone damper, or a refrigerant leak. The symptoms may include uneven temperatures, some zones blowing cold air while others are warm, or the system cycling on and off frequently.
Some possible solutions include checking the thermostat settings, inspecting the zone dampers for proper operation, and having a professional HVAC technician diagnose and repair any refrigerant leaks or other issues with the system. It's generally best to call a professional for any complex HVAC repairs, as they have the expertise and tools to properly identify and fix the problem.