The answer to 'ductwork smells bad' is that a bad smell in your ductwork is typically caused by mold, mildew, or bacterial growth. This can happen when moisture gets into the ductwork, often due to leaks, high humidity, or condensation buildup. The symptoms include a musty, unpleasant odor coming from the vents or air registers.
The best solution is to have a professional HVAC technician inspect the ductwork, identify the source of the problem, and thoroughly clean and disinfect the system. They may need to make repairs to stop any leaks or moisture issues. Homeowners should not attempt to clean ductwork themselves, as it requires specialized equipment and techniques to do it properly and safely.