PURPOSE: To make it possible to grasp a repair time of a heat exchanger with high accuracy by determining when to repair a multiple pipe heat exchanger for combustion air-preheating installed in an exhaust gas duct of a heating furnace based on a comparison between a stack draft force and a total of a pressure loss of an exhaust gas up to the inlet of the stack.

CONSTITUTION: The pressure loss of various exhaust gases in a gas duct 24 and a stack 23 is calculated from the input of combustion gas of a heating furnace 21 under combustion load and exhaust gas pressure on the inlet and outlet sides of a combustion air preheating multiple pipe heat exchanger and exhaust gas components and the temperature of a furnace bottom 22 of the exhaust gas. Furthermore, the total of the exhaust gas pressure from the draft force of the stack 23 including the resistance in the stack 23 up to the inlet of the stack 23 is calculated based on the result of the pressure loss calculation. When the comparison between the total loss of the exhaust gas pressure up to the draft force of the stack 23 and the inlet of the stack 23 is smaller than a predetermined value, it is judged that no draft allowance exists so that the heat exchanger must be repaired. This construction makes it possible to grasp when to repair the heat exchanger 25 with high accuracy.