All forced air or warm air furnaces have a filter that stops dirt from entering the fan and heat exchanger. This keeps the system clean by removing some of the dirt and lint that moves with the airflow of the furnace.
The filter must be changed when it is dirty or blocked with lint. This must be done on a routine basis. The frequency depends on how often the furnace (or central air conditioning) is operating, the type of filter, and the amount of dirt in the air stream.
One key to replacing the filter is the airflow direction. The outer frame of the filter is marked with an arrow indicating proper airflow direction. This arrow must match the airflow in the furnace return duct into the furnace blower or fan.
The airflow direction of the filter is important because the filter medium – fiberglass or a type of paper – must be properly supported by its wire mesh or paper frame. The filter support prevents the filter medium from being sucked out of the frame and into the furnace fan. If a filter plugged the fan, this could make the furnace overheat or it could damage the fan motor, requiring an expensive repair.
