A dirty furnace filter can reduce airflow, increase run time, and show a visibly dust-loaded surface. Check it with the system off and compare it with a clean filter or the manufacturer’s criteria; do not wait until it is completely black before replacing it.
Inspect the filter from the user-serviceable access
Turn the system off and remove the filter according to the manual. If the incoming-air side is coated and the filter media is barely visible, it is heavily loaded.
Lower airflow and longer cycles can accompany restriction
Rooms that take longer to heat or weak airflow at vents can accompany a loaded filter. If a correct clean filter does not improve the issue, the cause lies elsewhere in the system.
Check size, rating, and direction
A filter that is too restrictive, undersized, or installed backward can also cause problems. Follow the airflow arrow and the rating allowed by the equipment manufacturer.
Adjust inspection frequency to household conditions
Pets, construction, smoke, and heavy dust load filters faster. Inspect more often in those conditions and use the manual interval as a baseline rather than a guarantee that every filter lasts the same time.
Sources
- U.S. Department of Energy — Energy Saver — Practical information on heating, filters, home comfort systems, and energy efficiency.