How Often Should I Check or Replace My HVAC Filter?

Why the filter matters

A forced-air system pulls room air through a filter before heating or cooling it. As the filter collects dust, airflow through it drops. A clogged filter can make the system work harder and can let dirt build up on internal parts.

Finding your filter

Turn the system off at the thermostat before removing a filter.

• Behind a large return-air grille on a wall or ceiling (often in a hallway).
• In a slot on or next to the furnace or air handler.
• Some homes have more than one return, and more than one filter.

Reading the label

Filters are printed with their nominal size (for example, 16 × 25 × 1) and an airflow arrow. The arrow points toward the furnace or air handler — in the direction the air travels. Write the size somewhere you'll see it, such as inside the grille or on your maintenance record.

Filters also carry an efficiency rating, commonly a MERV number. Higher ratings capture smaller particles but can restrict airflow in some systems. If you're considering a higher-rated filter than you currently use, check your equipment manual or ask an HVAC technician whether your system is designed for it.

A routine that's easy to keep

• Pick a fixed day each month to look at the filter.
• Hold it up to a light. If you can't see light through it easily, or it looks gray and matted, replace it.
• Keep a spare or two of the correct size on hand.
• Note the date on the filter's frame with a marker when you install it.