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All gas and oil furnaces have temperature switches that control when the blower starts and stops. These allow the burners to heat up the air to a given temperature before the blower starts to circulate the heated air through the ducts. This minimizes the possibility of cool initial drafts when the furnace starts, but it also wastes heat. Much of the heat during this no-blower start-up time is lost in the basement or utility room..
The same thing happens when the furnace shuts off. The blower continues to run until the air temperature drops to a predetermined level. Then the blower stops and much of the heat remaining in the furnace heat exchanger is just lost in the basement. On many older furnaces, this shut-off temperature is set to about 120 degrees.
Next time you have your furnace cleaned, have the serviceman adjust the switches so that the blower starts sooner and shuts off later. It is not a difficult adjustment to make. Have the serviceman show you how he does it so you can adjust it yourself in the future. Remember, whenever working on any appliance that uses electricity, always SWITCH OFF THE ELECTRICITY AT THE FUSE OR CIRCUIT BREAKER BOX FIRST.