A heating and cooling filter is dirty when dust buildup begins restricting airflow, not simply when it looks slightly gray. The most reliable check is to remove it as directed, look through the media, and compare it with a clean filter. Reduced airflow, longer run times, and icing can accompany restriction, but those signs can have other causes too.
Inspect the filter with the system off at the control
Open only the user-serviceable filter slot or cover. If the surface is covered in compact dust and barely passes light where it once did, it is time to replace or clean it according to the filter type.
Do not wash a disposable filter that is not designed for it
Disposable pleated filters can deform or lose performance when wet. Reusable filters have specific washing instructions and need to dry fully before reinstallation. Follow the airflow arrows when putting the filter back.
Judge dirt by airflow conditions, not only a calendar
Pets, construction, smoke, pollen season, and operating hours greatly change loading rate. Inspect more often during demanding periods and adjust the interval to your home and manual instead of assuming every filter lasts the same number of months.
Icing or low airflow with a clean filter needs another diagnosis
If a new filter does not improve airflow, there is icing, unusual noise, or the system cannot reach temperature, do not keep swapping filters. Ductwork, the coil, blower, or refrigerant system may be involved and require appropriate service.
Sources
- U.S. Department of Energy — Energy Saver — Practical information on home heating, cooling, thermostats, maintenance, and efficiency.