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How to Change a Furnace Filter (and How Often)

Updated November 2, 2026 · General information, not HVAC or gas-fitting advice

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The short answer: Turn the system off at the thermostat, slide out the old filter, and put in a new one of the same size with the airflow arrows pointing into the duct, toward the furnace. PNNL says to replace it every 3 to 6 months. ENERGY STAR says to inspect, clean or change it once a month. A dirty filter can cut airflow and strain the furnace.

Why it matters

The DOE building science center says a dirty or clogged filter can dramatically reduce airflow, increasing furnace run time, wear on the motor and energy consumption. ENERGY STAR says a dirty filter can raise energy costs and damage equipment, leading to early failure.

How often

Source	Advice
PNNL Building America Solution Center	Replace the filter every 3 to 6 months.
ENERGY STAR	Inspect, clean or change air filters once a month.
DOE building science center	Replace or clean frequently; no set interval.

The sources differ. Our suggestion: look at the filter monthly, change it when it looks dirty, and follow the filter's package and your furnace manual.

Steps (PNNL)

Find the filter slot. It is usually between the return air duct and the air handler, or in the first few feet of ductwork. Some homes have the filter at the return grille.
Turn off the HVAC system at the thermostat.
Open the slot cover and pull out the old filter. Handle it carefully: PNNL warns the metal wire over the filter fabric can be very sharp.
Check the size. The new filter should cover the duct opening completely and sit flat, not diagonally. Match the length and width on the old filter's frame.
Remove plastic wrap but keep the cardboard frame. Write the date on the frame.
Install it with the airflow arrows pointing into the duct, toward the furnace.
If the filter is loose, PNNL says to seal gaps with foam weatherstripping around its frame. Do not tape or caulk the slot cover.
Close the cover and turn the system back on at the thermostat.

Which MERV?

The EPA says to choose at least MERV 13, or as high a rating as your system fan and filter slot can accommodate, and that you may need an HVAC technician to determine the highest rating that suits your system. PNNL says higher MERV filters can restrict airflow more and to check with your HVAC service company before going above MERV 12. ServiceMag says a dense 1-inch high-MERV filter can trigger limit trips. So the sources disagree on the number. Check the maker's manual or ask your HVAC company before moving to a high rating.

Need a new filter?

Match the size printed on the old filter's frame. Check your furnace manual before choosing a high MERV rating.

How to change it See furnace filters

Common questions

How often should I change my furnace filter?

PNNL says every 3 to 6 months. ENERGY STAR says to inspect, clean or change it once a month. Follow the filter's package and your furnace manual.

Which way do the arrows point?

PNNL says the airflow arrows should point into the duct, toward the furnace.

Can a high MERV filter hurt my furnace?

It can restrict airflow. PNNL says to check with your HVAC service company before going above MERV 12, and ServiceMag says a dense 1-inch high-MERV filter can cause limit trips.

Related guides

Sources