A: Dear SH, The Chimney Balloon inflates into place so the balloon membrane only goes out as far as the flue wall allows it to. See attached images of a Chimney Balloon inflated in a 8" ID clear tube. This allows the balloon to fill a round or square area.
For a tube type fireplace flue that is circular in shape, you would
measure the diameter of the flue and buy a Chimney Balloon that is the same size or slightly larger. So an 8" or 9" diameter flue would require a 9x9
Chimney Balloon and a chimney flue that is 10" to 12" in diameter would
require a 12x12 Chimney Balloon, and so on...
Most round chimney flues are between 6" to 12" in diameter, so 95% of the time a 9x9 or 12x12 Chimney Balloon will match the application on a round flue. - Jason