I'm replacing a prefab fireplace circa mid 1980's and my chimney sweep said the inside dimension of the current (air cooled?) pipe is 8". Can I install a new 6" (inside diameter) ASHT pipe inside the old pipe or will the old pipe need to be ripped out? The pipe dimensions in the installation manual show that a 6" ASHT pipe is 8" in diameter on the outside so I'm guessing it will have to be replaced. The model pipe I may be using is: Security Chimney's 6" LD Secure Temp ASHT.