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Have an old CG, 20"x30"CC w/o a SFB. Stack is 3"x9"above and am extending it down to grate level , which adds another 9'''. After interior mods( bricks, radiant deflector)with no or light wind's-runs great-and have been happy with cooks. Now fall winds are showing themselves and Mondays cook(winds-15+), even with usual go to's ( shelter, positioning, makeshift chimney shield) she seemed to need more draft. So, look at Feldons, calculations are with SFB. Tried to search for some kind of formula for sizing and am getting frustrated trying to match one with a smoker. Am using a coiled flashing extender ( 1 1/4 wrap ) in chimney and that holds pretty tight to 2 3/4" - 3". Could the interior obstructions be slowing draw thus having the wind cause an even more effect, not showing up in zero to light winds. Not getting a backdraft situation but just seems like i need more draw. When i hit it with a small usb fan at the intake things ran normal. Was thinking of using a 3" x 4" smoke increaser with a 4" smoke 90 on top to help suck air and articulate 90 to a 45 or straight depending on wind. Any thought's or suggestions???? Wind will blow from the NW at my house till spring and would like to get on this ASAP since i'm back on 12's for a while. Thanks.