Friend has an existing factory built 6" stainless steel chimney, currently being used for a wood stove. He would like to swap out the wood for a pellet stove. Any reason or code that says you cannot do this? Also if allowed what type of pipe would you use to connect the pellet stove to the chimney? Regular black 4", typical pellet vent pipe with some sort of adapter to go from the pellet to chimney connector??? Thanks for any help.