How and when to check a forced-air furnace filter
A filter check takes minutes, but the correct direction, size and replacement interval matter.
Look at the filter regularly and replace it when it is visibly loaded or when the equipment or filter manufacturer says it is due. Match the printed size and type, and point the airflow arrow toward the furnace or air handler.
Finding and reading the filter
The filter is commonly in a rack beside the furnace or air handler, or behind a return-air grille. Turn the system off at the thermostat before sliding it out. Note the printed dimensions and the direction of the airflow arrow before removing it.
Do not assume a thicker or more restrictive filter is automatically better. The filter needs to suit the rack and the airflow the system was designed to move.
There is no universal calendar
Filter life changes with filter area and type, fan runtime, pets, renovations, smoke, dust and how tightly the home is closed. Check more often during heavy heating or cooling use and during dusty work. Follow the equipment and filter manufacturers’ instructions rather than relying on a generic number of months.
What a dirty filter can affect
A loaded filter can restrict airflow. That may contribute to weak airflow, uneven comfort, longer run times, overheating or coil icing depending on the system and season. Replacing a dirty filter is sensible maintenance, but it will not correct every airflow or equipment fault.
Call if the pattern is unusual
A filter that becomes heavily loaded very quickly, collapses in the rack, whistles, gets wet or never seems to collect anything deserves a closer look. There may be a sizing, bypass, duct or moisture issue.