I am lining the chimney for installation of a hearthstone pheonix. The total height of the chimney from stove to the top is 18'. I had 25ft of oval flex liner to put in there, but after the first 13ft the 8x12 terra cotta flues change direction. I had to use 5" round pipe for the last 6 or so feet. I will be insulting to entire length. Does this sound OK?