The short answer
Turn the system off, find the filter at the furnace or the return grille, note the size printed on its frame, slide in a new one with the arrow pointing toward the furnace, and turn the system back on.
Steps
- Turn the thermostat off (or to fan off) so the blower doesn't pull dust in.
- Find the filter: a slot beside the furnace or air handler, a cabinet on the side, or behind a return grille in a wall or ceiling.
- Pull the old filter out and read the nominal size printed on the frame (for example 16x25x1).
- Slide the new filter in with the arrow pointing toward the furnace, in the direction the air flows.
- Close the door or grille and turn the system back on. Write the date on the frame.
How often
| Filter | Typical change |
|---|---|
| 1-inch pleated | Every 1 to 3 months |
| 2-inch pleated | Every 2 to 4 months |
| 4- to 5-inch media | Every 6 to 12 months |
Change more often with pets, during renovation, in smoke season or when the system runs heavily.
Several returns
Some homes have a filter at more than one return grille and none at the furnace, or both. Check every return before you buy, and buy each size you find.
Products mentioned
Questions readers ask
Can I run my furnace without a filter?
Only briefly; dust coats the blower and coil quickly. Put a filter back in as soon as you can.
What happens if the filter is in backwards?
The pleats can bow and pull loose and dust can bypass the media; turn it around so the arrow points toward the furnace.