A turn down ratio damper for controlling an amount of gas and air flowing in the TDR damper and delivering the controlled gas and air to a turbo fan, the TDR damper comprising: air passages comprising a first air passage and a second air passage, the first air passage and the second air passage separately formed so that the air moves through each path; gas passages comprising a first gas passage and a second gas passage, the first gas passage and the second gas passage separately formed so that the gas moves through each path; and opening and closing means (250, 260) for opening and closing the second air passage and the second gas passage at the same time, characterized in that the air passages and the gas passages are separately formed and reach outlets connectable to the turbo fan so that the air and gas can be delivered to the turbo fan through a separate path, wherein when air flows in from the rear of the TDR damper and gas flows in from the lower part of the TDR damper so as to discharge the air and gas through the outlets disposed on the front of the TDR damper, the first air passage and the second air passage are straight-lined pipes and the first gas passage and the second gas passage are curved pipes, the air passages and the gas passages connected to the outlets in the same direction, wherein the second gas passage comprises a via hole formed on one side of the curved part and the opening and closing means comprises: a gas opening and closing part, which opens and closes the second gas passage; and an air opening and closing part, which opens and closes the second air passage, wherein when the opening and closing means are to close the second gas passage and the second air passage, the opening and closing means move in a direction of the outlets so that the gas opening and closing part is inserted into the via hole so as to close the second gas passage and simultaneously, the air opening and closing part close the entry of the second air passage, and when the opening and closing means are to open the second gas passage and the second air passage, the opening and closing means move in an opposite direction of the outlets so that the gas opening and closing part opens the second gas passage and simultaneously, air opening and closing part opens the entry of the second air passage. air passages comprising a first air passage and a second air passage, the first air passage and the second air passage separately formed so that the air moves through each path; gas passages comprising a first gas passage and a second gas passage, the first gas passage and the second gas passage separately formed so that the gas moves through each path; and opening and closing means (250, 260) for opening and closing the second air passage and the second gas passage at the same time, characterized in that the air passages and the gas passages are separately formed and reach outlets connectable to the turbo fan so that the air and gas can be delivered to the turbo fan through a separate path, wherein when air flows in from the rear of the TDR damper and gas flows in from the lower part of the TDR damper so as to discharge the air and gas through the outlets disposed on the front of the TDR damper, the first air passage and the second air passage are straight-lined pipes and the first gas passage and the second gas passage are curved pipes, the air passages and the gas passages connected to the outlets in the same direction, wherein the second gas passage comprises a via hole formed on one side of the curved part and the opening and closing means comprises: a gas opening and closing part, which opens and closes the second gas passage; and an air opening and closing part, which opens and closes the second air passage, wherein when the opening and closing means are to close the second gas passage and the second air passage, the opening and closing means move in a direction of the outlets so that the gas opening and closing part is inserted into the via hole so as to close the second gas passage and simultaneously, the air opening and closing part close the entry of the second air passage, and when the opening and closing means are to open the second gas passage and the second air passage, the opening and closing means move in an opposite direction of the outlets so that the gas opening and closing part opens the second gas passage and simultaneously, air opening and closing part opens the entry of the second air passage.