To replace a furnace filter, turn off the power to the unit, open the filter access panel, remove the old filter, and slide in a new one. The filter is typically located between the return air and blower, but the exact location may vary by model.
Before replacing the filter, check the filter access panel for the filter size or part number. This information may also be found in the manual or on the filter itself. With the part number or filter size, visit a home improvement to purchase a replacement.
Before removing the old filter, turn off the power to the furnace. The electrical switch may be located near or on the unit. Look near the return air and blower for the filter access panel. Open the panel, and lift or slide out the old filter. Filters have directional arrows; note which way the old filter was inserted.
Unwrap the new filter, and insert it into the access panel in the same direction as the old one. Replace the filter access panel, and restore power to the furnace.
The filter replacement frequency varies by model, so review the owner's manual for specific recommendations for your unit. Some filters can be replaced annually, while others require more frequent maintenance. If using an electrostatic filter, it should be cleaned every month during heating season.