Well, my brother-in-law likes my homemade "garnlike" boiler that I made a couple of years ago which is approximately 3000 gallons so he want to build a bigger one for himself. He has a 7 foot diameter tank by 21 feet long. I think that the tag says it holds 5000 imp gallons. I think we will use 6-inch pipe for the exhaust pipe and a 16-inch pipe for the reaction chamber and maybe a 42-inch pipe for the firebox. I believe that there will be approximately 100 feet of pipe for the heat exchanger. Depending what the exhaust temperature is, we could add a tubulator in the last pass. I'm a little worried about condensation forming in the last pass if I used multiple smaller pipes for the last pass since it is so long. I added one to mine this year and it seemed to reduce the temps at least 100 degrees at the exhaust. Also, I want to maybe build on Tom's idea about bringing in air to the front of the unit in two places for the primary and secondary air. Still thinking about how to do that. I love the simplicity of these units. Any ideas?