[elhigh]

I like to draw house designs for fun; a big solid masonry chimney is a typical defining feature of most of my designs. Like your sewer pipe example, mine is for structural and thermal service.

I don't see any major headaches with wrapping tubing around the vent pipe - you might have to clean your chimney an extra time or two per heating season, but that's about it.

The heat shield on the back of the heater would be completely replaced by the heat exchanger - while the heat exchanger is pulling heat from the stove, the plate would probably be safe to touch - hot, like a teapot: you can touch it, but not for long. And it wouldn't set anything on fire.

Try laying a lot of Styro slab insulation directly on your crawlspace floor. It ain't cheap, but once it's paid for, it's paid for. The dollars that go into heating it go right back out again, soaked into the ground under it.