I am installing a small wood furnace in my basement. It was made by an Amish metal shop and is very heavy built. It is a little bigger than a standard wood stove. The fire box in 12" wide, 24" long, and about 18" high. The firebrick is 9 1/2" tall. The flue outlet is 8". This seems to big to me and want to know if I can use a 6" chimney. I am planning on using Dura Vent insulated pipe. The flue will run through the main floor, and out the roof. Total flue not including stove pipe will be about 18 feet. Any suggestions? I noticed the new woodstoves at rural king are about the same size and they use 6 inch. Thanks for any advice, Terry