from Doug in Stevens Point, WI:
#72
Should I close off vents in rooms that I am not heating/cooling?
No, especially if the room has a return air duct.
No—especially if you are closing off a room that has a return air duct. A return duct sucks air out of the living space and returns this air to the furnace or air conditioner. The air is then mixed with fresh air, reheated or cooled and redistributed to the living space.
A room that is closed off will have an unusually low or high temperature compared to the rest of the home. Pulling this extreme temperature into the furnace or air conditioner will make the unit work much harder to condition the air before distributing it to your home. Also, closing vents will restrict airflow coming out of the furnace or air conditioner, which will cause the blower motor to either work harder to overcome the barrier, or distribute less air which will affect the comfort level of the rest of the home.
When you close off an unused room, the extra energy that the equipment requires to operate will actually lower the efficiency of the unit (because it is using more energy) and could cause premature failure of the heating or cooling components.