Q: Jason, The dimensions you give for Chimney Balloon sizes appear to be for rectangular or square openings. What size do I use if I have a 9 inch round diameter pipe? – SH

A: Dear SH, The Chimney Balloon inflates into place so the Chimney Balloon membrane only goes out as far as the flue wall allows it to. See attached images of a Chimney Balloon inflated in an 8″ ID clear tube. This allows the Chimney 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 9×9 Chimney Balloon and a chimney flue that is 10″ to 12″ in diameter would require a 12×12 Chimney Balloon, and so on…

Most round chimney flues are between 6″
to 12″ in diameter, so 95% of the time a 9×9 or 12×12 Chimney Balloon will match the application on a round flue. – Jason