I have searched the site for people that have installed inserts in old fireplaces that have Heatalator boxes installed and have seen what they have done. It seems that you have to cut the damper open to get the chimney liner in. I have seen some inserts that have the flue on the top of the stove. My damper is towards the rear of the box and I do not think it can make the bend to connect the flue. Do they make an adapter for such a thing or should I be looking for a certain stove that would make a better connection to the flue pipe? The height of the opening is 26 1/2" and it slopes down in the back. I am not looking for a big insert since I have a pellet stove in the basement just want one that will heat about 1000 sq ft when the power goes out or just to take the chill off on a really cold days. I do have an old Quaker Moravian insert there now that does a good job.