Replacing a furnace filter is usually a pretty simple task, once you know what you’re doing. There are major issues to pay attention to, for example buying the wrong size furnace filter or putting it in backwards. Doing this incorrectly, you could end up bypassing the flow of air instead of cleaning it. Here’s how to do the job easily.

1. First shut off the power to your furnace.
2. Remove the panel enclosing the filter (usually located inside the furnace or inside the return air duct near the furnace).
3. Slide the old filter out towards you. Look for an arrow on the old filter indicating airflow direction.
4. Using a permanent marker, draw the airflow direction on the outside of the filter cover, so you’ll always know the right way to install the filter. This arrow should be pointing toward the furnace, which is the direction of the airflow. It is a good idea to also mark the filter size, which will be printed on the cardboard frame.
5. Then to install the new filter; look for the markings that tell you the direction of airflow, and slide it in with the arrow facing the furnace (as previously marked).
6. Once you slide the filter back into place reinstall the cover that was removed and refasten it. It is a good idea to keep a record of the date so that you’ll know when it’s time to change the furnace filter again. It never hurts to keep a spare filter nearby.