I am getting a new insert for my fireplace in a few days. The existing chimney is straight up 21' with no bends, meaning I could drop a straight pipe straight down the chimney into the fireplace. The dimensions of the chimeny are 12X12 I believe and the dimensions of the firebox are about 33X33X24 (yes, I know, way to huge) so I have plenty of room to work. The chimeny runs up a outside wall of the house and is constructed of brick. The whole wall that the fireplace is in is concrete block with no combustables. Could I just use standred 6" black stove pipe wrapped with insulation for a liner? I really don't want to buy a flexable one or spend the money on ss. Thank you