How to change a furnace or AC filter
To change a furnace or AC filter, switch the system off at the thermostat, find the filter slot in the return air grille or beside the furnace, read the size printed on the old filter's frame, and slide in a new one with the airflow arrow pointing toward the furnace. Most homes need a new filter every one to three months.
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How to do it
- Set the thermostat to off so the system does not pull dust in while the slot is open.
- Find the filter: behind a large return grille on a wall or ceiling, or in a slot on the side of the furnace or air handler.
- Slide the old filter out and read the three dimensions printed on its cardboard frame.
- Buy the same size, in the filter type your system's manual recommends.
- Find the arrow on the new filter's frame and point it in the direction the air flows, toward the furnace.
- Slide it in fully, close the grille or cover, and switch the system back on.
- Write the date on the frame and set a reminder for the next change.
Common mistakes
- Putting the filter in backward, against the airflow arrow.
- Buying a filter that is the wrong size and leaves gaps round the edge.
- Choosing the densest filter available, which some systems cannot pull air through.
- Forgetting it for a year until the system strains.
Quick self-check
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Teach: How to change a furnace or AC filter
What to say
To change a furnace or AC filter, switch the system off at the thermostat, find the filter slot in the return air grille or beside the furnace, read the size printed on the old filter's frame, and slide in a new one with the airflow arrow pointing toward the furnace. Most homes need a new filter every one to three months.
What to show, in order
- Set the thermostat to off so the system does not pull dust in while the slot is open.
- Find the filter: behind a large return grille on a wall or ceiling, or in a slot on the side of the furnace or air handler.
- Slide the old filter out and read the three dimensions printed on its cardboard frame.
- Buy the same size, in the filter type your system's manual recommends.
- Find the arrow on the new filter's frame and point it in the direction the air flows, toward the furnace.
- Slide it in fully, close the grille or cover, and switch the system back on.
- Write the date on the frame and set a reminder for the next change.
The check question
Ask: Which way should the airflow arrow point?
Related skills
- How to test smoke and carbon monoxide alarms
- How to reset a tripped breaker or GFCI outlet
- How to winterize outdoor faucets
- Every skill in Home Fixes
Sources
- Your heating and cooling system's owner's manual, for your model. Checked September 28, 2026.
Last reviewed . First published .