Changing the furnace filter is the single easiest thing you can do to keep your heating and cooling running efficiently and your air cleaner. A clogged filter strains the system, raises bills, and can cause it to overheat and shut down.
Find and replace it
The filter sits in a slot near where the return duct meets the furnace, or behind a return-air grille. Note the size printed on the frame, slide the old one out, and slide the new one in with the airflow arrow pointing toward the furnace.
How often
Check it monthly during heating and cooling season and replace it when it looks grey and dusty — typically every one to three months. Homes with pets, allergies, or construction dust need more frequent changes.
Choosing a filter
Filters are rated by MERV — higher numbers trap finer particles but restrict airflow more. A mid-range MERV balances air quality and airflow for most homes. Very high-MERV filters can strain some systems, so match the filter to what your furnace can handle.