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Know your system ยท Filters

Air filters: where they are, how to check them, when to change them

Short version: the filter in your furnace or AC protects the equipment and the air you breathe. It is usually at the furnace or behind a return air grille. Check it each month with a light test, replace it when light no longer passes through easily, and use the right size. If nothing else, changing it every 3 months is a good rule of thumb.

FIELD STUDY / 17Four steps to check your filter
Four steps to check your filterFind the filter at the furnace or in a return grille, turn the system off at the thermostat with the fan on AUTO, do the light test, and use the right size. Check monthly; every 3 months is a good rule of thumb.1Find it: furnace or grille2System off, fan on AUTO3Light test4Right size: measure itCheck monthly; change by 3 months
Schematic field study. Not to scale or an installation plan.

Where the filter is

The filter is usually in one of two places: in a slot at the furnace, or behind a return air filter grille on a wall or ceiling. A return grille is a vent that pulls air back toward the system.

How to check it

Before checking an accessible filter, turn the system off at the thermostat and make sure the fan is set to AUTO, not ON. Do not shut it off at the furnace switch or the electrical panel for this. Then do a light test: hold the filter up to a light beside a new one. If light passes through easily, air can too. If the light looks dim, or does not come through at all, replace it. Color alone does not decide its condition.

FIELD STUDY / 17How to check your filter
How to check your filterStep 1: the thermostat is set to OFF with the fan on AUTO, not ON. The furnace switch and breaker are not used for this. Step 2: side by side, the two usual filter locations: a slot at the furnace, where the filter slides out, and a return air filter grille on a wall or ceiling, whose door swings open so the filter comes out. Step 3: the light test: a lamp shines behind a new filter and the used one; light passes easily through the new filter and barely through the used one, so it needs replacing. Step 4: a new filter of the correct size goes back in at both locations.OFFFan AUTOSystem OFF, fan on AUTOAt furnaceIn a grilleFurnace or grille: take it outNewUsedDim light? Replace itAt furnaceIn a grilleRight size goes back inNewUsedLight test: new vs. used

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Read the field notes
  • Turn the system off at the thermostat and make sure the fan is set to AUTO, not ON. Do not use the furnace switch or the breaker for this. Do not reach inside the furnace.
  • The filter is usually in a slot at the furnace or behind a return air filter grille on a wall or ceiling.
  • Light test: hold the filter up to a light beside a new one. If light passes through easily, air can too. If it looks dim, or no light comes through, replace it. Color alone does not decide its condition.
  • Every home is different. Check monthly to learn how fast yours gets dirty; every 3 months is a good rule of thumb. The size printed on the old filter can be wrong. To find the correct size, measure the space the filter fits into, look for a label at the furnace or grille, or ask a pro.
Schematic field study. Not to scale or an installation plan.

How often to change it

How long a filter lasts is different in every home. Check it each month to learn how fast yours gets dirty, and you will know how often to change it. If nothing else, changing it every 3 months is a good rule of thumb. FilterPing puts check and change dates on your own calendar.

Getting the right size

The size printed on the old filter can be wrong. To find the correct size, measure the space the filter fits into, look for a label at the furnace or grille, or ask a pro.

Which rating to buy

MERV is the number that says how well a filter catches particles. Higher catches more, but a filter your system cannot push air through can cause its own problems. The MERV guide explains what the EPA suggests and when to ask a technician.

Where to go from here

If the airflow still seems weak or the system still struggles after a fresh filter, then it is recommended to have a pro evaluate the situation and mention what you have done so far. This will help get the problem solved quicker.

Sources

ENERGY STAR suggests checking the filter every month, especially during heavy-use months, and changing it at least every 3 months. The light test, filter locations, size advice and shutoff steps come from field experience. Checked October 28, 2026.

When in doubt, call a pro. If you are not comfortable checking something, or you are not sure, please reach out to a pro. Some of these issues can be made worse, more expensive to fix later, or dangerous.