I'm in the midst of designing my garn like boiler. I have a 2800 gal tank that is 7 foot in diameter and 10.5 feet long. I figure that I can store heat in the water to last at least 24 hours or about 1.4Mbtu's. The total heat storage is 2800 gal * 8.33 lbs/gal *(180-120 degrees F). What I'm trying to figure out is how to calculate my firebox so that my burn time to heat 1.4Mbtu's will only take 3 hours (about 500Kbtus). I can calc the wood required (lbs) but I'm not sure of the right air (cfm) to get the 500kbtu/h burn rate. Once I know this, I can calc my exhaust hx size. The air required to burn completely (Stoichiometric) plus the excess air 40-100%??? I may have to dig deep in the old thermo book. If anyone has suggestions, I'm all ears.