I have recently purchased a BK King Ultra. I am going to be installing the chimney myself. With my install I must go through a wall into a chase then through the roof. I am planning on using double walled stove pipe to the thimble then double walled insulated chimney pipe for the rest. My question is: I know in the manual it says there must be 36" of vertical before the 90. In my case, with vaulted ceilings, I can go from 8' to 15' before the 90. Is more than the required 36" better or worse for draft? The house is two stories and the chimney is almost in the middle of the house so I am guessing the total height of the stack will be around 20' or so. Any help with this would be greatly appreciated.