The main reason to adjust the flue or dampers on your home furnace is to control airflow through the system's heating ducts. Opening or closing the dampers can allow you to adjust airflow through the system as it travels to different rooms of your home. According to mechanical engineer and home improvement radio show host Tom Feiza, opening the furnace dampers to encourage the intake of cold air during the winter months can help heat your home more efficiently because it removes existing cold air from your home in favor of heated furnace air.
During the winter months, closing or restricting the second-floor dampers of your duct system can allow your home to retain more heat. Restricting the dampers keeps more of the heat from your furnace directed to your first-floor rooms and encourages the heat to rise to the second floor. This allows your furnace to heat your home with less mechanical effort because the system isn't constantly pumping heated air up to your second floor only to have it rise up and out of your home.
If you have a central cooling system powered by your furnace, dampers play an integral role in keeping your home at a reasonable temperature throughout the summer. Completely opening your second-floor dampers helps stored heat escape from your home and lowers the overall temperature inside. This reduces the stress on your furnace to create cool air. Partially closing your first-floor dampers helps keep the lower rooms of your home cool while avoiding turning your living room and dining room into a blast chiller from a constant barrage of cold air.
Regularly cleaning your flue or damper system along with your furnace filter helps keep your furnace running at peak performance levels. This practice also removes potentially hazardous buildups of soot or debris, which can catch fire or clog the system resulting in a higher heating/cooling bill because your furnace has to work harder to create a comfortable living environment. A severely clogged furnace filter can damage the internal components of your furnace resulting in thousands of dollars worth of repair bills.