How to change your furnace or AC filter

Last reviewed November 1, 2026 by Jordan Reeves

Turn the system off, slide out the old filter, and put in a new one of the same size with the airflow arrow pointing toward the furnace. ENERGY STAR says to check it every month and change it at least every 3 months.

How often: Check monthly. Change when dirty, and at least every 3 months. Thicker 4-inch filters can last longer, up to 6 months according to Carrier. Your filter's label decides.

Steps

Turn the system off at the thermostat.
Slide out the old filter and read the size printed on its frame. Buy that exact size. If nothing is printed, measure the slot and round up to the nearest inch.
Find the airflow arrow on the new filter. It points toward the furnace or air handler, away from the return. At a wall or ceiling grille, that means into the wall or ceiling.
Slide it in, close any cover, and turn the system back on.

Stop and call

Call an HVAC technician: if the old filter was black or matted after a short time, you find no filter at all, the filter is in a ceiling grille you can't reach from a step stool, or the unit is in the attic.

Why it matters

ENERGY STAR says a dirty filter slows air flow, makes the system work harder, wastes energy and can lead to early system failure.
Heating and cooling account for nearly half of a home's energy use, according to ENERGY STAR.

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Sources

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