There are many reasons for reduced furnace performance but a common cause is, in a word, dirt. When the furnace’s heat exchanger or blower wheel gets coated with dust, efficiency is reduced. In the case of the heat exchanger, even a thin layer of dirt will act as an insulator, reducing the efficient transfer of heat to the air that flows by it. A dirty blower uses more amperage and pushes less air. Your electric bill increases, less heat (or cool air in summer) gets to the rooms of your home, and components fail sooner than they should.
A dirty furnace can lower your heating system’s efficiency significantly and waste hundreds of dollars annually. Have your furnace cleaned and tuned up annually and be sure to replace furnace filters regularly.