The answer to 'zone control system smells bad' is that a foul odor coming from your zone control system is typically caused by mold, mildew, or bacterial growth within the system. This can happen if there is excess moisture or humidity in the ductwork or air handlers. Common symptoms include a musty, unpleasant smell coming from the vents or air registers.
To address this issue, you'll want to first inspect the system and identify the source of the odor. This may involve checking the air filters, coils, and drip pans for any signs of microbial growth. You can try cleaning these components with a mild disinfectant solution. However, if the problem persists or the odor is particularly strong, it's best to call in an HVAC professional. They have the tools and expertise to properly diagnose and resolve the issue, including addressing any underlying moisture or ventilation problems that may be causing the odor.