1.   Go in the basement and inspect the round ducts leading to rooms from a larger rectangular trunk duct.

2.   If there are no dampers in the round ducts, you will have to make do with closing the vents in each room affected.

3.   In your system is a large duct where hot air is stored, with smaller, round ducts coming off it, leading to the rooms.

4.   Round ducts extend from this trunk to the registers in each room.

5.   The connection between round duct and trunk duct is trickier, but you should be able to figure that out by looking at it.

6.   Where the round duct leaves the big trunk duct, you may see a little metal lever.