i Want to put a wood stove in my basement and reuse the old oil fired furnace chimney, when the previous owner put an addition on the house he capped off the chimney at the roof line. The Harmon wood stove requires a 6 inch flue which is the same size as the clay liner inside the chimney, however it looks not in good condition lots of cracks and looks like some has worn away., This being said I don’t trust it , worried about chimney fire and possibly leaking smoke in house, so I took out the old clay liner , good thing too, in some spots it was less than 1/8 inch thick, This left the chimney block and an approx 9 inch opening , I am considering installing a class a double wall chimney in the existing masonry the run and extending the masonry above the roof line using the 10-3-2 rule The run is will be about 25 feet and straight up . Is this doable, and how is it supported inside the masonry, Any advice u can give would be great, Thanks