How to change your furnace and AC air filter

Updated October 10, 2026. By the OakTend team.

Your furnace and central AC pull air through one filter, usually in a slot at the furnace or air handler, or behind a return grille in a wall or ceiling. Changing it means swapping in a clean one of the same size.

How often: ENERGY STAR says to check it every month and change it when it looks dirty, or at least every 3 months.
What you need: New filter in the same size, Flashlight, Marker

Why it matters

A clogged filter chokes airflow, so the system works harder and cools or heats less. Filters matter most here on wildfire smoke days, when the EPA suggests a MERV 13 filter, running the fan on On instead of Auto, and setting the system to recirculate.

Steps

Turn the system off at the thermostat.
Find the filter: in a slot at the furnace or air handler, or behind a return grille.
Slide the old filter out and read the size printed on its edge.
Note the airflow arrow on the frame. It points toward the furnace or blower, away from the return grille.
Slide the new filter in with the arrow the same way. It should fit snugly, with no gaps around the edges and no bending.
Write the date on the frame, close the slot or grille and turn the system back on.

When to call a pro

Ask an HVAC technician before moving to a high-MERV filter on an older system with a 1-inch slot. The Department of Energy notes a thick high-MERV filter can restrict airflow there, and a lower rating may be the better fit.

Safety

Turn the system off before you open the filter slot so the blower is not pulling in unfiltered air and dust. The EPA notes a filter only cleans the air while the system is running.

Go deeper

General information, not professional advice. Your equipment's manual comes first when it says something different.

Sources

ENERGY STAR: a guide to energy-efficient heating and cooling
Check the filter every month and change it if it is dirty, or at least every three months; the filter may be in the duct system rather than the equipment.
EPA: guide to air cleaners in the home
Choose MERV 13 or as high as the system allows; the filter should fit snugly without bending; it filters only while the system runs.
EPA: wildfires and indoor air quality
During smoke, use a MERV 13 filter, set the fan to On instead of Auto, and use recirculate.
U.S. Department of Energy: guide to HVAC system maintenance (Building America)
In older air handlers with 1-inch filter slots, high-MERV 1-inch filters may block airflow.