Step 4: Clean or replace the old filter. With the filter out, you have a choice. You can use a vacuum to clean the old filter. If it's made of plastic filtration material, you can also wash it and reuse it. However, you may wish to buy a replacement filter, which is inexpensive. Slide the new or cleaned filter into the furnace, making sure to read any markings on the filter side to ensure proper installation.
Step 5: Replace the filter door and turn the furnace on. Screw or snap the filter door back into place. Flip the emergency switch to the "on" position. Your furnace will now be operating at a more efficient level.