Dirty AC or furnace filters are the number one problem when forced-air heating and cooling systems don’t work right.

With a central forced-air heating and cooling system, heat-pump, furnace or AC air filters remove dust and allergens from the air before warming or cooling air and returning it to your rooms. For this reason, filters are critical components of these systems.

But, on the downside, as AC, heat-pump, or furnace filters become clogged with dust and debris, they block the free flow of air, drastically reducing the system’s efficiency. Properly maintaining furnace or AC filters is an important way to keep your heating and cooling as affordable as possible. Clean permanent filters or replace disposable forced-air system filters every six months unless they become clogged with dust sooner.

Filters are typically located at the home’s ceiling or wall return-air registers, and/or they may be placed in the furnace or AC unit’s air-handler cabinet.

Position the replacement filter in the wall register with the airflow arrows pointing in toward the ductwork. ©Don Vandervort, HomeTips

Before beginning filter replacement, turn off the heating or cooling system. After you locate replaceable filters, note the sizes marked on their frames or measure their sizes.

filter size markings
Keep images of your home’s filter sizes on your cell phone for reference when buying replacements.

How to replace a filter in a room’s return-air duct register:

1 Unlatch the register’s cover grille and swing it out of the way or remove it.

Note: On many registers, the “hinge” is made to come apart, so be careful that the grille doesn’t detach and fall off.

Remove the old filter and immediately put it in a trash bag or the outdoor trash.

changing furnace filter
Carefully unlatch grille’s cover and remove the dirty filter. Steve Heap / Shutterstock.com

2 Clean dust off of the return-air’s grille before installing the replacement filter. Use a damp rag to clean the grille and the surfaces of the register—both inside and out. Again, be careful: The grille’s hinge side may unlatch on some types!

cleaning furnace filter grille
Clean dust off of the grille. Steve Heap / Shutterstock.com

3 Position the new replacement filter in the register with the airflow arrows pointed in toward the ductwork. Replace the grille and latch it.

Mark the date on the filter’s frame so you’ll know when it’s time to change it. (The filter shown is being installed in a wall return-air register.)

replace furnace filter
Replace the filter with “air flow” arrow is pointing toward ductwork. Mark the date on the filter’s frame so you’ll know when it’s time to change it. ©Don Vandervort, HomeTips

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How to replace a filter in a furnace or air-handler cabinet:

1 Turn off the power to the furnace or air handler. Look for the door or panel that conceals the blower; sometimes this is marked “Filter.” Lift this door or panel off of its holding hooks or unscrew its retaining screws to remove it.

service person doing forced air furnace care
Remove the air handler’s door (often by lifting it). Lennox

2 Check the filter. A standard filter is mounted next to or under the blower motor. Slide the filter out along its tracks.

furnace forced air heating
Furnace Parts Diagram

Check to see whether it is a disposable filter or intended to be cleaned and replaced—this should be marked on the filter’s edge, along with directions for cleaning, if applicable.

If it’s a disposable filter, its size will more than likely be printed on the frame’s edge.

3 Buy a replacement filter and slide it into place, noting the arrows stamped on the side that indicate the proper direction of airflow; be sure you face these toward the blower (away from the ductwork). Then replace the door to the cabinet.

The video below offers good advice for changing furnace and AC filters:

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About Don Vandervort
An avid builder and remodeler, Don Vandervort has developed his expertise for more than 30 years, as Building Editor for Sunset Books, Senior Editor at Home Magazine, author of more than 30 home improvement books, and writer of countless magazine articles. He appeared for 3 seasons on HGTV’s “The Fix,” and served as MSN’s home expert for several years. Don founded HomeTips in 1996. Read more about Don Vandervort