I recently purchased a home (my first and first fireplace) with what I've been told is steel heatilator fiireplace. My guess is it's thirty plus years old in poor condition (rotted firebox). My goal is to install a stove insert and build a decorative mantle that would cover the old heatilator vents in the existing masonry. My questions are: 1. Can I safely cover the vents (or block them) with an insert application. a. Will heat still be cycled through vents. 2. How much space should be left between the perimeter of insert (outside face) and any woodwork. I hope my description is clear. Thanks for any advice.