The answer to 'zone control system smells bad' is that a smelly zone control system is usually caused by mold, mildew, or bacteria buildup in the ductwork or HVAC components. This can happen when there is excess moisture or poor airflow in the system. Common symptoms include a musty, unpleasant odor coming from the vents or registers. To solve this, you'll need to thoroughly clean and disinfect the system.
If the smell persists or you're unsure of the cause, it's best to call in a professional HVAC technician. They can inspect the system, identify the source of the odor, and recommend the appropriate solution, which may involve more extensive cleaning, repairs, or even replacement of components.