The sun is the source of all life on earth, emitting radiant energy constantly since billions of years. Trees use this energy together with water, CO2 and minerals to grow. Chopping the trees, splitting and cutting the timber to stack and season it eventually lead to firewood, pure stored sun energy.
Burning timber in a clean combustion at very high temperatures between 800°C and 1.000°C sets free all the stored sun energy, some water vapour, the same amount of CO2 which the tree used to grow and leaves back ash containing mainly minerals which have proven to be a good fertilizer in the garden.
The heat is stored in the thermal mass of the kachelofen so that the combustion air, the CO2 and the water vapour leave the top of the chimney with about a 100°C which prevents condensation and creosote build up. The far majority of the energy is emitted over the surface as natural and healthy radiant heat just like sun rays the following hours.
Mark Twain says about heating with a kachelofen: “This noble stove is at its very best when its front has a big square opening in it for a visible wood fire. The real heating is done in the hidden regions of the great structure […].”