Question: We are in the process of building a new house with a double sided wood & gas fireplace. The flue is 10", so it's commercial grade. Our contractor walked out, and it's up to us to finish the house. He left a hole in-between the rooms where the fireplace is to go, and a hole in the rafters. The dry wall is up, and there is no chimney yet. (Sounds ridiculous doesn't it.) Our contractor walked off with most of the money, and we have to close on the loan. I have a double sided gas log set purchased. (The plumber will be running the gas line not us.) I don't have to tell you what we are talking about price wise to purchase a CLASS "A" chimney that is 10". If we only use gas logs and NEVER burn wood, can we install a CLASS "B" type chimney? We have NO BUILDING CODES where I live, just the national codes apply. Besides suing my contractor, can you lead us in a direction if we can't use CLASS "B"? We are real handy, and are planning on doing most of the work ourselves. Answer: Assuming that you have installed a pre-fab firebox, you MUST use the matching pipe that is the same brand as this firebox. You will be happy to find out that this type of metal chimney costs a lot less than the generic class A chimney that you may have priced. Since this chimney matches in a certain way to the firebox, you cannot substitute anything else, including B vent or Class A. If the fireplace "box" was site-built out of masonry, then you should use the 10" class A chimney and install it on an anchor plate which affixes to the top of the masonry enclosure. I think it would be a mistake to use B vent for a few reasons. 1. Most gas logs are approved to install ONLY into vented wood burning fireplaces. This means the chimney has to be capable of wood burning. 2. Someday, when you sell the house, there could be problems if the new owners want to burn wood.