PURPOSE: To provide a fast idling operation of the internal combustion engine and to prevent an overheating of a waste gas type heat exchanger by a method wherein an output from the waste gas type heat exchanger using cooling water for the internal combustion engine and waste gas heat is circulated to the cooling water type heat exchanger having its water feeding operation terminated.

CONSTITUTION: When a sensor 11 for sensing a temperature of cooling water in the internal combustion engine detects a water temperature lower than a predetermined value, two-way valve 12 is closed, a distributer three-way valve 13 is changed over to close a flow passage toward the secondary hot water piping 6, and then the hot water in the waste gas type heat exchanger 3 is flowed to the distributer three- way valve 14. Thus, the water feeding to the cooling water type heat exchanger 2 is shut off, and in place of this operation the hot water in the waste gas type heat exchanger 3 is supplied, so that the cooling water for the internal combustion engine is heated in the heat exchanger unit 2. And also the idling of the internal combustion engine is rapidly made and an absorption heat in the waste gas type heat exchanger unit 2 is taken out for heating of the cooling water, resulting in that the hoverheating may be prevented.