This pre-fab fireplace needs to be replaced. The refractory panels inside the firebox are cracked, the doors do not operated smoothly. The refractory panels inside the firebox are cracked, and the sliding fire screens are bent and do not slide; the glass doors do not close easily.
Here is a top view of the flue pipe of the system. Notice that there a pipes within pipes. This is what makes the system "zero clearance" which means the parts of the system can be installed within one inch of combustible materials. The air space between the pipes creates buffer zones of multiple spaces of air that rise and vent out the top.
When a system has been over-fired, these pipes can buckle and separate ( The pipes are put together in sections). Usually you cannot disconnect the sections once they have been locked together (twist-locked). This is safety design. When pipes buckle and warp they need to be replaced. Many manufacturers have since gone out of business, so getting parts is almost impossible. Adding after market parts will void the UL listing of the system. The bottom line is that the whole system needs to be replaced.
Here is a system that was over-fired (notice the holes on the top of the rain cap.) The flat, rusted rectangular cover is a chase cover. The chase is a wood box that the chimney pipe travels through. Chase covers can be replaced easily.
Here is a new unit being installed into the wall (rear view.)
Here is an exposed chase at the roof.
Video of an installation project.