Lowtech Graph lists low-tech options for every need of a home. Here you can compare them, find tutorials to build them, and link them together into a circular system. Why low-tech?
The heat still in the smoke as it leaves a stove for the chimney: most of what a stove loses. A flue coil or a heat reclaimer can catch some of it, as long as the flue stays hot enough to keep tar from forming.