The answer to 'why is my dryer vent stained' is that a stained dryer vent is usually caused by lint buildup or clogging in the vent. As hot air and lint from the dryer cycles through the vent, it can leave behind a visible stain or discoloration over time. This staining is a common sign that the vent needs to be cleaned.
Staining can also be caused by moisture or condensation in the vent, which can lead to lint and debris sticking to the interior walls. If the vent has any bends or kinks, this can restrict airflow and cause lint to accumulate, leading to stains. Regular cleaning and maintenance of the dryer vent is important to prevent this issue.