I have a pellet stove insert that was stuck into an existing gas fireplace. The pellet stove is routed about a foot into the existing chimney with a metal plate to block off the rest of the chimney around it. Existing was 8 inch and pellet stove is 3 inch. Now all of this was done before I bought so I'm assuming this is code. I would really like to swap to a wood burning insert. The 8 inch pipe(cant tell if it's single, double wall etc.) runs straight up in a chase, through the an overhand, through the attic and out the roof. Its in a 2 foot by 5 ft chase so I think the clearances would be ok. Could I run a 6 inch pipe through the 8 inch and in essence make a double wall pipe? Or Should i swap it all out for Class A? Thanks for any help and advice.