If you’re a first-time owner of a home with a forced-air furnace, you might not realize there’s a filter in there that needs changing. And even if you do, you might not know it needs to be replaced every few months to keep your furnace running at peak efficiency—and to help prevent pollutants from entering your living space. 

Fortunately, replacing a furnace filter is a pretty simple task, once you know what you’re doing. There are ways to botch the job, however, for example by buying the wrong furnace filter or putting it in backwards, which could block the flow of air instead of cleaning it. Here’s how to do the job properly in three simple steps.

Step 1

What kind of filter do you have? Start by turning off the furnace. Remove the existing furnace filter, which will be located inside the furnace or inside the return air vent. Look for an arrow on the filter indicating airflow direction. Using a permanent marker, draw the airflow direction on the outside of the furnace, so you'll always know the right way to install the filter. Then note the furnace filter size, which will be printed on the cardboard frame.

Time out! A filter that has a plastic frame is a reusable model. That means you have clean it only periodically with a vacuum and water, ideally outdoors. Let it dry completely before reinserting.

Step 2

Get the right replacement. Furnace filters are sold at home centers, hardware stores, and online. Disposable ones are typically 1 or 2 inches thick. Check our Ratings of furnace filters, also known as whole-house air filters, for a right-sized model that was effective at removing dust, pollen, and smoke when air passed through it at both high and low speeds. We also test thicker furnace filters, some up to 5 inches, and they often provide superior air cleaning. But if your furnace isn’t already equipped to handle a thicker filter, it will need to be modified by an HVAC professional.

Our top-rated 5-inch furnace filter is the Lennox Healthy Climate CarbonClean 16, $100. For 1-inch furnace filters, we recommend the Filtrete Healthy Living Ultimate Allergen Reduction 1900 MPR, $20. For more choices and sizes, see our full Ratings and recommendations of furnace filters.

Step 3

Install the new filter. Look for the markings that tell you which side of the filter should face the furnace. Then slide the filter back into place and replace any cover that goes over it. Keep a record of the date so that you’ll know when it’s time to change the furnace filter again.