The answer to 'HVAC system smells bad' is that there are several common causes for a foul odor coming from your HVAC system, including mold or mildew buildup, dead animals, and rotting debris. Symptoms may include a musty, rotten, or burning smell. Solutions can range from simple DIY cleaning to professional duct cleaning or repairs, depending on the severity of the issue.
If the smell is mild, you may be able to resolve it by changing the air filter, cleaning the air ducts and vents, and running a disinfectant through the system. However, if the odor is strong or persistent, it's best to call in an HVAC professional to diagnose and address the root cause, which could require more extensive cleaning or repairs.