A first period during which drain is not substantially generated on a surface of a water tube (4) of a heat exchanger (B1), and a second period as a transition during which an amount of drain on the surface of the water tube (4) is increased are provided. There are a plurality of kinds of fan-controlling data in which operation control contents of the fan (13) in the first and second periods in correspondence with change of conditions of entering water temperature or combustion level of the burner (1). The fan-controlling data are stored in a data table (90) of a controller (9). The controller (9) controls the fan (13) in accordance with the data, and an initial air-fuel ratio of combustion driving of the burner (1) is stabilized.