Step 1
Shut off power to the furnace. A breaker switch is typically built into the side of the furnace, or you can cut off the unit at the household circuit breaker.
Step 2
Open the blower compartment on the side of the furnace, using a screwdriver to turn the screw latches that release the vented cover. On some models the screws may not come out completely. Instead, twist each screw half a turn counterclockwise to disengage the latches and open the cover.
Step 3
Pull out the old filter and check the printing around the frame for the dimensions, for example, "12 x 24." If the dimensions are not printed on the old filter, measure the part with a tape measure and write down the length and width in inches to purchase the correct Naturalaire replacement..
Step 4
Insert the new Naturalaire filter into the compartment with the arrow on the edge of the filter facing inward toward the blower.
Step 5
Close the cover and turn the screw latches to hold the cover in place.
Step 6
Restore power to the furnace.