How often should you change your furnace filter?

A $15 filter protects a $6,000 system. Here is how often to swap it, and which one to buy.

Homeowner sliding a clean pleated furnace filter into a furnace beside a dirty one

A clogged filter is the most common cause of the no-heat calls we get in November. It makes your furnace work harder, raises your gas bill and can trip the safety switch that shuts the system down.

The quick answer

1-inch filters: every 1 to 3 months
4- to 5-inch media filters: every 6 to 12 months
Homes with pets or allergies: check monthly

Which filter to buy

For most homes, a MERV 8 to 11 pleated filter is the sweet spot. Higher ratings catch more dust but can restrict airflow in older systems.

How to change it

Turn the system off, slide out the old filter and note the arrow on the frame. Insert the new one with the arrow pointing toward the furnace, then write the date on it.

Set a reminder on your phone for the first of every other month. Your furnace will thank you.