Filters that are clogged are the most typical source of furnace troubles. Dust and grime obstruct airflow and if the filter becomes too clogged, the heat exchanger overheats and shuts down too rapidly, preventing your home from warming up. Replace the filter if the blower is operating but no heat is going out. A clogged filter also causes soot accumulation on the heat exchanger, lowering the furnace's efficiency and shortening its life.