In order to achieve an optimum combustion with optimally low fine particle content at a heating apparatus for solid fuel with the combustion chamber and secondary combustion chamber, the primary trigger of the flue gases is optimized from the combustion chamber into the afterburning chamber and simplified by selecting the appropriate shape of the hollow profiles (2), between which the primary trigger done to improve especially the formation of sweeping swirls of gases passing therethrough on the back of the hollow sections (2).