The answer to 'why is my air duct stained' is that air duct staining is a common issue caused by a buildup of dust, debris, and even mold or mildew. Over time, the airflow through the ducts can carry these particles and deposit them on the duct surfaces, leading to visible staining and discoloration.
Some common causes of air duct staining include poor air filtration, high humidity levels, and inadequate duct cleaning. Symptoms may include visible stains, musty odors, and reduced airflow. Solutions can range from basic duct cleaning to more extensive duct replacement, depending on the severity of the issue.