The answer to 'why is my air duct grimy' is that air ducts can become dirty and grimy over time due to a buildup of dust, debris, and even mold. This can happen for a few common reasons, such as poor air filtration, infrequent cleaning, and even pests or animals nesting in the ducts.
The symptoms of grimy air ducts include reduced airflow, musty odors, and visible dirt or grime buildup. To address this, you can try basic duct cleaning methods like vacuuming or using a duct cleaning tool, but for more extensive buildup, it's best to call in a professional air duct cleaning service to thoroughly clean and sanitize the system.