What does a dirty furnace filter look like and how can I identify it in my HVAC system?

A dirty furnace filter may appear gray or dark in color, and it may be covered in dust and debris. To identify it in your HVAC system, you can visually inspect the filter for dirt buildup or use a flashlight to see if light passes through easily. Additionally, a dirty filter may cause reduced airflow and decreased heating or cooling efficiency in your home.