How often should I replace my furnace or AC air filter

A one inch thick filter rated MERV 8 to MERV 11 needs replacement every month or two in average homes, while a four to five inch thick filter can go three months. If you have pets, smokers in the home, or live in a dusty area, replace more frequently. Check your filter by looking at it every month from the front of the slot or case.

How to know it's time

Hold the filter up to a window light. If you cannot see light clearly through it, it's clogged and restricting airflow. A clogged filter makes your system work harder, raises energy bills, and can trigger nuisance shutdowns if airflow gets too restricted. Some thermostats display a "change filter" alert if the system detects high resistance.

Where to buy and how to install

Buy filters at hardware stores, online retailers, or from your HVAC company. Note the dimensions printed on your current filter frame. Changing a filter takes two minutes: turn off your system, slide out the old filter noting the airflow direction arrow, insert the new filter with the arrow pointing toward the furnace or air handler, and turn the system back on. This is the single best maintenance task you can do yourself.