I bought a new Fire Chief furnace to add on to my existing gas furnace ductwork in my unfinished basement. Question I have is in regards to the flue. I want to run it up in the center of my house , which is the corner of a room on the main floor. I want to run the flue vertical about 12'' away from my main floor support beam. The book says to stay 18'' away from combustables with your single wall pipe. I am going to run single wall from the furnace to the floor, then into a S-1 ceiling support, then class a chimney straight up. Can I put durrock and metal around the beam and be safe? I would use class A on down but I have to use a ceiling support conector to support all the class A pipe.