PURPOSE:To constitute the device inexpensively so as to have a small size without deteriorating the heating capacity thereof by a method wherein a tap-controlled heating unit is provided with a high-pressure small flow amount type independent blower, sending primary combustion air, and a low-pressure large flow amount type common blower, sending secondary combustion air separately, and the primary and secondary combustion airs are sent into a normal heating unit by the common blower. CONSTITUTION:In case a hot-water supplying machine is used, a fuel intercepting valve only is opened and fuel is supplied to the burner 4 of the same machine in accordance with the necessary load thereof. At the same time, the primary combustion air of the optimum flow amount is sent by the independent blower 16 and the primary combustion air of constant flow amount is sent by the common blower 17. In case a bath boiler is used, the fuel intercepting valve only is opened to supply the fuel of constant flow amount to the burner 7 for the bath boiler. At the same time, the primary and secondary combustion airs of constant flow amounts in accordance with the flow amount of fuel are sent by the common blower 17 and the water, being supplied into a heat exchanger 9 for the hot-water supplier, may be heated effectively.
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