I just have to inject this. My old insert was the same age as yours. Heated this joint for 21 years. The last year of its life I lined the chimney and put a boot on it to connect the liner. Not knowing that a crack had developed in the back. Combined with the fact that the stove was never envisioned to have the kind of draft that liner provided, and the crack letting air in, the sucker ran away the first time it was fired after the liner install and got up to 1,400 degrees stove top and scared the hell out of me. Plugged the crack with furnace cement and the next night the same thing happened again. Be careful giving those old inserts too much chimney draft that they never dreamed of when they designed them to have since they thought they would be dumping into flue tiles. My liner is 21' tall. Just sayin... .