3 years ago I purchased a unit and it worked perfect, the next year I put a shed around it and put a chimney on it and thats when the problems began. There is a backdraft so bad I can feel the draft through the fan in the door, this causes the fire to go out. The chimney I put on is the same as what is coiming out the unit, its 8" diameter stainless steel, the pipe I got was a 10' long piece, I welded on a 4" strip on the end and stacked them up. The only way to keep the fire going is to keep the door cracked open a little. After awhile the chimney gets completely blocked with creosote and soot so bad the only way to get it out is with a crowbar from inside the unit. It seems the blockage is where the two pipes meet. Is there air getting in at the seem causing the backdraft or does the chimney need to be higher, its already about a foot below the peak but its about 12' from the peak, I also capped it , no change. Does the chimnet need to be insulated? I am to the point of taking down the chimney, so any help I would appreciate.