Ok here is my two pennies worth of input. As a general rule you want double the inflow as you want the outflow. Since you are using 3 inch pipe you have about 7 sq inches, so you have those numbers if your inflow is 15. What the excell sheets dont tell you in the gap on the end of your plate which should be the size of the outflow (in area) times 1.3, so it should only be about 10 sq inches. If I had to take a guess, the amount of airflow you have is too high, and your energy is going right out the pipe and not into the metal. One way to maybe measure that is to mount a temp probe in the higher section of the cook chamber and see what the air flow temp is looking like. Which brings me to another question. You said the length of your stack, but is the inlet to the stack at the top of the cook chamber or down by the first grate level? You are also a little skinny on heat mass. (Thin Metal) It's going to take more energy to keep the system warm. I didnt read what type of fuel you are using, but you might try a higher grade of coal or wood. Just make sure if you are using splits that they are fine splits or pizza wood. It will be more efficient for a smoker that size.
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