Abstract: A segmented radiant burner assembly for installation in the combustion chambers of firetube boilers and the like. The individual segments can be assembled together for on-site installation within the boiler. The segments include support structures comprising mounting flanges which are secured together in series to form the burner assembly. Gas and thermal sealing between the active material of the burner and the inactive support structure is provided by a sealing system which includes a dense fiber composition material and adhesive agent of ceramic composition. The mixture of unburned reactants is directed through a valving and manifold system into separate plena within the burner segments. The flow rates of reactants to the different burner segments is selectively controlled in a sequence which achieves a broad firing rate range.