PURPOSE: To prevent the overheating of an economizer by providing a control section which supplies to the main body of a boiler the water that has become hot in the economizer and replenishes water of low temperature to the economizer by driving a supply water pump.

CONSTITUTION: A control section 7 is provided with a temperature sensor 10 which is installed at a spot that is most liable to be overheated through the water flow channel of an economizer 2. When a temperature detected by the temperature sensor 10 rises over a set temperature, the control section 7 opens an automatic blow valve 6 and drives a supply water pump 4 to discharge the boiler water of the boiler main body and supplies the high temperature water of the economizer 2 into the boiler main body as the boiler water in the main body 1 is discharged and also replenishes the economizer 2 with the low temperature water in a supply water tank 3 to control the temperature of the water in the economizer. In the meanwhile, the carry-over of the boiler water in the boiler main body 1 can be prevented positively by opening the automatic blow valve 6 for a specified time or until the water level in the boiler main body 1 goes down to a specified height after blow and driving the supply water pump 4 for a specified time or until the water level in the boiler main body 1 reaches a specified operation height after lowering once the water level in the boiler main body.