How to Change a Furnace Filter

Airflow from the vents has gone weak, the furnace runs long and shuts off early, or a year has passed and nobody remembers the last change. A filter is there to protect the blower and the heat exchanger, not to purify the house, and a filter installed and forgotten is one of the most common reasons a furnace overheats and shuts itself down.

Turn the furnace off at the service switch

Flip the switch that looks like a light switch on or near the unit, or the furnace breaker. The blower should not be pulling air through an open slot while a hand is in it.

Find the filter slot

Most filters sit in a narrow slot between the return duct and the blower compartment, usually at the bottom or the side of the furnace behind a removable cover. In houses with a central return, the filter instead lives behind a large grille in a wall or ceiling, held by a hinged frame.

Read the size printed on the cardboard frame

The size is stamped along the edge of the old filter, such as 16x25x1. Nominal size differs slightly from the actual measurement, so buy by the printed number rather than by a tape measure. Note the number somewhere before the old filter goes in the trash.

Point the arrow toward the blower

Every filter frame carries an airflow arrow. It points in the direction the air travels, which is away from the return duct and toward the blower and furnace. Installed backwards, the media collapses against its own support and restricts airflow. Slide the new filter in flat, with no gaps around the edges, and close the cover.

Choose a MERV rating the system can handle

MERV 8 through 13 covers what a house needs, capturing dust, pollen, and most of what a residential system should be asked to catch. Higher ratings restrict airflow, and many furnaces and older duct systems cannot move air through them without straining the blower. Check what the furnace manufacturer allows rather than buying the highest number on the shelf.

Set a replacement interval and write the date on the frame

A one inch filter needs changing every one to three months, and monthly during heating season is reasonable for a house with pets or anyone in it with allergies. Four and five inch media filters run six months to a year. Construction dust or a recent renovation shortens all of those.

If the furnace has been shutting down on its own. A restricted filter trips the high limit switch, which cuts the burner while the blower keeps running. Replace the filter, then watch a full heating cycle.

If there is any smell of gas. Leave the building, do not touch switches or use a phone indoors, and call the gas utility or 911 from outside.