My house has a classic, wood-burning fireplace.  No fans, no nothin' to push warm air back into the house.  As a result, it makes the area immediately in front of the fireplace a cat-magnet, the remainder of the living room pleasant, and drops the temperature of the rest of the house by a few degrees (3°F, if you believe Mythbusters).  All that heat leaving my house annoyed me.

I started thinking about to recover it.  I had a brilliant insight!  The chimney is basically a metal pipe running inside the masonry.  Why not wrap the chimney in a water-jacket, or pipe, and send the heated water through a radiator?

Turns-out I'm not the first to think of this.  Also turns-out it doesn't work.  The interweb says that (1) there isn't enough heat in the chimney to be usefully extracted; (2) if I pull heat from the chimney, I'm reducing the energy that is propelling combustion products out of the building -- which is another way of saying that I'm increasing the soot & smoke in my living room, and raising the probability of a chimney fire.  <sigh>  Yet another brilliant theory undone by physical reality.

On the other hand, I did accidentally learn about firebacks.  These are either big, heavy cast-iron plates that sit in the back of the fireplace and re-radiate absorbed heat, or lighter, stainless-steel sheets that reflect heat back into the room.