Down on the farm (in our case, that little bit of heaven is located in upstate New York), winters get so harsh that icicles form on the inside of the windows just to get out of the cold! So a few years back, inspired by a particularly chilly spell, I set out to find a way to squeeze a little more heat out of the aged hardwood logs that were rapidly disappearing into the gullet of our Glenwood Oak wood stove.
The result was a squared circle of stove pipe. I can't claim credit for designing the BTU scavenger, though, because I got the idea after noticing a similar rig in the background of a photo in an old issue of National Geographic.
The square is made from standard eight-inch stove pipe: two T's, four 90 ° elbows, and a pair of straight sections. The damper is located above the assembly, and from there the pipe exits through the ceiling by way of an insulated, triple-wall thimble. All of the sections are fastened with sheet metal screws for security.
And—for safety's sake—let me remind you that any device which salvages heat from a stove pipe, as ours does, is going to encourage the formation of creosote ...and that could lead to a chimney fire. To prevent such an occurrence, I clean the inside of the assembly whenever the sooty buildup gets to be more than 1/4" thick, and replace all the pipe every second year. The latter may be an expensive practice, but good accident prevention is about the best insurance bargain I know of!