PURPOSE: To obtain the max. recovering quantity of exhaust heat in any operational level by enabling pressure loss control of a heat exchangers arranging flue of each hot stove and air piping for combustion and controlling the pressure loss of the blast exchanger.

CONSTITUTION: Each the heat exchanger 5, 6 for recovering the exhaust heat, which is possible to control the pressure loss, is arranged at the flue 4 of the hot stove 1 and the air piping 3 for combustion. In these heat exchangers, each of the heat exchanging tube is rotatably arranged and used to be changeable to the pressure loss by changing rotating angle of fin in the tube. Sensible heat in combustion exhaust gas in the combustion pressure of the hot stove 1 is recovered by the exchanger 5, and when the pressure in the stove 1 is sufficiently high, by increasing the pressure loss in the exchanger 5, 6, the flow rate of the exhaust gas into the stove 1 is adjusted by flow rate adjuster 8, 9 of fuel gas and air for combustion. On the other hand, when the pressure in the stove 1 is insufficient, by decreasing the pressure loss in the exchanger 5, 6, the flow rate of the fuel gas and the air for combustion is adjusted.