A cracking furnace for the pyrolysis heating of an organic feedstock includes a heating section and at least one convection section. In one embodiment the furnace includes first and second convection sections positioned along opposite sides of the heating section. The openings for admitting flue gas to the convection sections can be at the top or the bottom of the heating section. In another embodiment the furnace includes a plurality of passageways for the communication of flue gas from the heating section to the convection section. The passageways can be positioned at the top or the bottom of the heating section. The passageways provide a more even flow of flue gas through the heating section by preventing recirculation of the flue gas within the heating chamber.