The answer to 'HVAC system smells bad' is that there are several common causes, including mold/mildew buildup, dead animals, and backed-up drainage. Symptoms include a musty, rotten, or foul odor coming from the vents or unit. Solutions can range from DIY cleaning to professional repairs, depending on the cause.
If the smell is musty or moldy, it may be due to moisture buildup in the ductwork or HVAC unit. This can be addressed by cleaning the coils, drip pans, and ductwork. However, if the problem is more severe, such as a rodent infestation or major mold growth, it's best to call a professional HVAC technician to properly diagnose and resolve the issue.