THREE PARTS OF THE AIR-MOVING SYSTEM SHOULD BE KEPT CLEAN: THE FILTER, THE BLOWER, AND THE BLOWER MOTOR. Dirt is the biggest enemy of your furnace. It can waste fuel and drastically lower efficiency. Dirt affects all three basic components of your furnace, so cleaning is the most important part of regular maintenance. The three parts of the furnace should be cleaned: the filter system, the blower, and the motor. The furnace filter should be replaced or cleaned at the beginning of the heating season and about once a month during periods of continuous use. To check the filter, take it out and hold it up to the light. If it looks clogged, replace it with a new filter of the same type and size regardless of the length of time it has been used.