How to Change a Furnace Air Filter
A clogged filter makes a heating and cooling system work harder and reduces airflow, and replacing one is among the simplest maintenance tasks in a home. The two details people get wrong are filter size and filter direction. Both are printed on the filter itself, so a photo of the old one prevents most mistakes.
The Quick Answer
Safety note
What You'll Need
- Replacement filter in the exact size your system uses
- Permanent marker — For dating the new filter
- Flashlight
- Screwdriver — Only if your filter cover is screwed shut
Step-by-Step
Step 1: Turn the System Off
Switch the heating and cooling system off at the thermostat before opening any panel or filter slot, so air is not moving through an open compartment.
Pro tip
If your thermostat has a fan setting, set that to off too, not just auto.Step 2: Find the Filter Slot
Filters sit in a slot at the furnace or air handler, or behind a large return-air grille on a wall or ceiling. Some homes have more than one. A flashlight helps in a basement or utility closet.
Pro tip
If you have never located it before, check where the large return duct meets the equipment.Step 3: Slide the Old Filter Out
Open the cover or grille and pull the filter straight out. Hold it flat so accumulated dust does not shake loose into the room.
Pro tip
Slide it into a bag right away if it is heavily loaded with dust.Step 4: Read the Size and Arrow on the Frame
The filter frame is printed with its nominal dimensions and an airflow arrow. Photograph both before discarding the old filter.
Pro tip
Nominal size and actual size differ slightly — buy by the printed nominal size.Step 5: Match the Replacement Filter
Use a replacement in the same size, and choose a filter type your equipment manufacturer supports. A filter that is too restrictive for your system can reduce airflow.
Pro tip
When in doubt, check the equipment manual or the label inside the furnace door.Step 6: Install It With the Arrow Pointing Correctly
Slide the new filter in with the airflow arrow pointing toward the furnace or air handler — the direction air travels, away from the return duct. Make sure it seats fully with no gaps around the edges.
Pro tip
A filter installed backward still filters poorly and can restrict airflow.Watch out
Do not force a filter that does not fit the slot; gaps let unfiltered air bypass the filter entirely.Step 7: Close Up and Restart the System
Refit the cover or grille securely, then turn the system back on at the thermostat and confirm normal airflow at the vents.
Pro tip
Listen for any new whistling, which can signal a poor seal at the filter slot.Step 8: Date the Filter and Set a Reminder
Write the installation date on the edge of the new filter with a marker, and set a calendar reminder to check it based on the manufacturer recommended interval and your household conditions.
Pro tip
Households with pets, allergies, or nearby construction usually need more frequent checks.
Tips
- Photograph the old filter frame before removing it; size and arrow direction are printed there.
- Set a recurring reminder rather than trying to remember by feel.
- Buy filters in the size your system specifies, and follow the manufacturer guidance for the filter type your system supports.
- Check the filter more often during heavy heating or cooling use, or with pets and renovation dust in the home.
Common Mistakes
- Installing the filter backward so the airflow arrow points the wrong way.
- Forcing an incorrect size into the slot and leaving gaps that let air bypass the filter.
- Choosing a very restrictive filter without checking what the system manufacturer supports.
- Leaving the system running while the filter slot is open.
Frequently Asked Questions
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