The most frequent cause of furnace issues is dirty filters. Airflow is impeded by dust and dirt, and if the filter becomes overly clogged, the heat exchanger overheats, shuts off prematurely, and your house doesn't warm up. Replace the filter if the blower is running but no heat is coming out. Additionally, a clogged filter contributes to soot accumulation on the heat exchanger, decreasing the furnace's efficiency and shortening its lifespan.