I have decided to go with an EPA Stove (really like the Quadra-fire 5700 Step top) rather than an add-on furnace. My chimney is exterior and has clay liners that have an iside diameter of 6-1/2" x 10-1/4". This stove has a 6" flue pipe. I called Hartshearth.com and spoke with a lady who said that the liner along with insulation would be to thick. They are to get back to me. What are my options here. Does it absolutely have to be insulated? Chimeny length is 16 feet.