I am looking to install the exhaust stacks today. Based on the calculator from DaveOmak, I have the heights of the stack required for various inner diameters. It seems you want to have this as close to 36" as possible? I infer this from his comment: If that is the case, and I don't have the appropriate sized round or square tube to get close to 36", is there any reason not to run two smaller stacks that add up to the appropriate volume? I have a bunch of square tubing, but I'd definitely need to run two stacks. If I run it through my calculator, with 4" square tubing, I would need a height of 86.54. I'd need 6" I.D. tubing to run a single stack, and the price goes up a lot as you get to the wider square and round tubes. However, lets say I have 4" square tubing, and I run two stacks, I only need a height of 43.27", which is a lot closer The closest would be 3 3.5" stacks. How much should I be concerned about being near that 36" exhaust stack height?