Replacing a furnace filter is pretty simple once you know what you’re doing. There are ways to botch the job, however—for example, by buying the wrong size filter or putting it in backward, which can block the flow of air instead of cleaning it. Here’s how to do the job properly.

What kind of filter do you have? Start by turning off the furnace. Remove the existing furnace filter, which will be 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.

Check to see whether the filter is reusable. A filter that has a plastic frame is a reusable model. That means you have to clean it periodically with a vacuum and water, ideally outdoors. Let it dry completely before reinserting.

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 for a right-sized model that’s effective at removing dust, pollen, and smoke when air passes 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 and long life. But if your furnace isn’t already equipped to handle a thicker filter, it will need to be modified by an HVAC professional.

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.