As Stewart, Glenn, Steve, myself, etc have said forges need larger than 8". A forge is not a furnace and a forge is not a decorative fireplace. Eight inches is standard for standard applications, your forge is not a standard application. Just because they build house chimneys does not make them knowledgeable about forge chimneys. Bottom line, Sorry to say but Stewart, Steve, and Glenn are right and your contractor wasn't. I am truely Sorry.