When it’s time to change your AC filter, around 1 to 3 months after you installed a new one, it should look dirty with an apparent build up of dust. If your filter looks clean and it’s been in place for the recommended time, you should check: How the filter is fitting into the holder: If the filter is loose or too small for the space, the air can circulate around it instead of going through it. If the filter is installed upside down: There is a correct air-flow direction for most air filters. Arrows indicated on the filter frame should point toward the fan. If the filter you’re using is the correct one