A clogged or backward filter can reduce airflow and add strain to an HVAC system. The arrow on the filter should point in the direction air travels toward the equipment.
Turn the system off
Set the thermostat off before opening the return grille. Note the filter size printed on the frame and photograph the old filter orientation if needed.
Inspect the filter
Hold it up to light and look for a loaded gray surface, collapsed media, or moisture. Never wash a disposable filter unless its instructions specifically allow it.
Install it correctly
Slide the new filter into the track with the arrow pointing toward the furnace or air handler. Close the grille securely and restore the thermostat setting.
Watch for ongoing problems
If the filter becomes dirty unusually quickly, airflow remains weak, or the system freezes, have the ductwork and equipment checked.