Replace the filter on your Carrier Performance 80 furnace at least twice a year, according to the manufacturer. Periodically installing a new disposable filter will prevent undue stress on the furnace; not to mention it will also keep your energy bills tame. Use the same type and size filter as the old one; compare them side by side when you pull the old one out just to be sure. Filter replacement on the Carrier 80 is simple.
Turn the electrical supply switch on the furnace to the "Off" position.
Open the filter cabinet door. There may either be a knob that you turn counterclockwise or a screw that you twist out with a screwdriver to release the door.
Grab the old filter with your hands and gently tug it out of the cabinet. Place it in a trash bag.
Install the new filter by simply pushing it into the housing where the old one was and close the cabinet door.
Turn the electrical supply switch to the "On" position to start your furnace again.