Abstract: A heating system for tall horizontal slot type coke ovens wherein the heating walls are provided with flues in which combustion occurs at multiple levels. The improvement lies in having a single flue stack of two channel design, each channel having port openings located at corresponding different heights or levels above the flue floor. When lead gas is employed for fuel, lean gas and air are supplied from different regenerator chambers to the separate channels respectively whereby a mixture of fuel gas and air is effected and combustion occurs at the port exit openings at each successive level. When rich gas is employed as a fuel, both regenerator chambers are employed to supply air for combustion through both channels of the stack and rich fuel gas is supplied by a separate distribution system to each vertical flame flue. Regulation of the flow of rich gas or lean gas from the two channels is provided by an adjustable brick means, such as a slide brick at the top of the two channels.