An in situ thermal system is disclosed for enhanced oil recovery and the like from a subterranean formation. The system pressurizes the formation with water whereupon the entire formation is heated to relatively high temperatures in the absence of gas formation to significantly decrease the viscosity of substantially all the crude in the formation and permit recovery thereof. A down hole, fuel fired radiant tube burner of long length is provided to achieve the desired heat patterns within the formation. The burner includes three concentric, closed end tubes with the innermost tube containing a burner and spaced closely adjacent the intermediate tube. A hole pattern in the innermost and intermediate tube establishes a laminar flow of the burner's products of combustion therebetween which modifies the radiation heat flux pattern so that uniform heat is emitted from the outermost tube along its length.