The answer to 'air duct smells bad' is that a foul odor in your air ducts is typically caused by mold, mildew, or bacterial growth. This can happen when moisture gets trapped in the ductwork, often due to leaks, high humidity, or poor ventilation. Common symptoms include a musty, unpleasant smell coming from your vents and potential respiratory issues for those with allergies or asthma.
The best solution is to have your air ducts professionally cleaned and sanitized. A qualified HVAC technician can inspect the system, identify the source of the odor, and use specialized equipment to thoroughly clean and disinfect the ductwork. DIY duct cleaning is possible but requires the right tools and expertise to be effective. It's generally recommended to call in a pro, especially for more extensive duct cleaning or if the problem persists.