Symptoms of a dirty air filter can include: Dust and debris accumulating on your intake vents Dust and debris accumulating near your output vents, often on the ceiling and walls nearby Dust accumulating on fan blades, curtain rods, on top of cabinets, indicating that there is an excess of dust in circulation Noticeable loss of cooling power in your AC system Sounds of excessive strain when your AC system starts up Longer periods of run time for you AC system to cool to desired temperature