The answer to 'why is my zone control system smells bad' is that a smelly HVAC system is often caused by a buildup of mold, mildew, or bacteria in the ductwork, vents, or other components. This can happen when moisture accumulates in the system, providing an ideal environment for these microorganisms to grow.
Some common symptoms of a smelly HVAC system include a musty or moldy odor coming from the vents, poor air quality, and potentially even health issues for those with allergies or respiratory conditions. The solution often involves a thorough cleaning and disinfection of the affected components by a professional HVAC technician.