It’s possible that it’s connected to a flue liner at the top that runs up the chimney. I would think you’d still be able to budge it somewhat though.

Pictures?

We also can’t see anything attaching when we stick our head in the box and look up the chimney. It’s a Sears Forester model number 143840600. It looks like this now. And I thought it was an eyesore before ☹️

Maybe try posting on hearth.com. Lots of knowledgeable people on the forum there.

I had an iron fire grate in my wood burning fireplace and we removed it when we switched to gas logs. The first time I tried to move that grate it was so heavy that I initially thought it was bolted to the floor. Just the grate alone was about 2-3 times the weight of an 8"x8"x16" cinder block, so that puts it at 80-120 lbs. Your fire box may just be a lot heavier than you think.

Bruce

FWIW, my Buck 91 wood stove insert weighs 620 pounds.

Did you ever get the insert removed? I have one as well and need to remove it

We finally got ours out! We opened up the fireplace pan and the cast-iron insert was bolted to the secured fireplace pan. Once we released the bolts there was a steel frame also holding them together that we removed. Once those were all taken apart the insert was free to move. Hope this helps!