The combined furnace consists of a heating insert with foundation combustion and ceramic smoke ducts, instead of an additional heating box between the heating insert and the chimney connection. In shaping these smoke ducts, we have a relative range of freedom. We just need to calculate them accurately, so that the exhaust gases do not cool down excessively and the water vapor contained in them does not condense in the chimney. In the long run, this can lead to clogging of the chimney by soot ointment. Ceramic smoke ducts accumulate heat received from exhaust gases and give it away more slowly, and thus much longer than in a typical convection oven. Actually, the combined furnace also works according to the principle of convection. However, if these additional smoke ducts are placed very close to the furnace wall., the share of convection in the heat given away will decrease and the level of radiation will increase.

The following terms refer to the upper sketch: 1 = heating insert, 2 = flue gas pipe, 3 = ceramic smoke ducts, 4 = visible furnace wall, 5 = wall insulation, 6 = air card.