Changing the furnace filter protects your furnace and filters the air, improving the air quality in your home. Disposable furnace filters have either a mesh pattern or a pleated accordion pattern. Replace these filters once a month to keep your furnace running optimally. If your system handles both heating and cooling, replace the filter monthly all year long.The filter catches the dust, dirt, pollen, carpet fibers, and pet dander in the air, which keeps the house cleaner and helps keep allergies at bay. It also keeps all that stuff from getting into the furnace, which helps the blower motor lasts longer and keeps compressor coils from clogging.