A method of using a heat exchanger for recovery of waste heat, which is configured to recover thermal energy of exhaust gas generated in a boiler and heat water, the heat exchanger comprising: a bottom plate configured such that an exhaust gas inlet is formed therethrough; a top plate configured such that an exhaust gas outlet is formed therethrough at a location opposite that of the exhaust gas inlet; a first side plate configured such that a plurality first side through holes is formed therethrough; a second side plate disposed opposite the first side plate, and configured such that a plurality of second side through holes is formed therethrough at locations opposite those of the first side through holes; a third side plate and a fourth side plate configured to connect the first side plate and the second side plate; a plurality of heat exchange tubes through which a fluid flows, and configured to connect parallel between the first side through holes and the second side through holes opposite the first side through holes; and fluid mixing headers installed on the first side plate and the second side plate, respectively; characterized in that the plurality of heat exchange tubes are formed as titanium material tubes, and the heat exchange tubes are fastened to the first side plate by inserting first ends thereof into the first side through holes and expanding the first ends, and are fastened to the second side plate by inserting second ends thereof into the second side through holes and expanding the second ends by a pressurizing device inserted into the first and second end of each of the heat exchange tubes and performing pressurization to expand the first or second end of each of the heat exchange tubes; and wherein the heat exchange tubes through which the fluid is supplied are grouped into a plurality of sections according to capacity of the boiler, wherein the method comprises supplying the fluid through all heat exchange tubes belonging to at least one section among the plurality of sections according to capacity of the boiler. a bottom plate configured such that an exhaust gas inlet is formed therethrough; a top plate configured such that an exhaust gas outlet is formed therethrough at a location opposite that of the exhaust gas inlet; a first side plate configured such that a plurality first side through holes is formed therethrough; a second side plate disposed opposite the first side plate, and configured such that a plurality of second side through holes is formed therethrough at locations opposite those of the first side through holes; a third side plate and a fourth side plate configured to connect the first side plate and the second side plate; a plurality of heat exchange tubes through which a fluid flows, and configured to connect parallel between the first side through holes and the second side through holes opposite the first side through holes; and fluid mixing headers installed on the first side plate and the second side plate, respectively; characterized in that the plurality of heat exchange tubes are formed as titanium material tubes, and the heat exchange tubes are fastened to the first side plate by inserting first ends thereof into the first side through holes and expanding the first ends, and are fastened to the second side plate by inserting second ends thereof into the second side through holes and expanding the second ends by a pressurizing device inserted into the first and second end of each of the heat exchange tubes and performing pressurization to expand the first or second end of each of the heat exchange tubes; and wherein the heat exchange tubes through which the fluid is supplied are grouped into a plurality of sections according to capacity of the boiler, wherein the method comprises supplying the fluid through all heat exchange tubes belonging to at least one section among the plurality of sections according to capacity of the boiler.