I see people building solar boxes and filling up the box w/ aluminum cans, dryer vent, etc. I don't get it. How is this more efficient than using a plain sheet of the foam board w/ the metal coating (or any kind of non-tubular flat surface)? Why would it be any better to run the air through a black tube (heating the air indirectly) than it would to heat the air directly under a glass panel? And it can't be a thermal mass for storage -- you would want any storage inside the home, not inside the furnace, right? I suppose corrugated metal increases your surface area, but even then, there is only so much energy shining into the window at any one time. AND ANOTHER THING! I've seen other people who build a furnace/can box, and then just tape it to an existing window. In the words of "Oh Brother..." "That don't make no sense. X amount of suns rays/energy/heat are entering that window whether there is a box or not. The advantage of a solar furnace is just that it, in essence, increases your home's window area -- more passive solar (albeit, without the heat loss of a regular window since it doesn't cut a massive hole in your house like a window does). I building a solar furnace for my chicken coop. Warm birds are laying birds.