PURPOSE: To supply continuously the necessary and sufficient air into a burner having even large passage resistance by a method wherein a blower for supplying a burning air to a burner is driven by a direct electric current motor.

CONSTITUTION: A turbofan 9 is provided in a blasting chamber 5 as a blower driven by a direct electric current motor 8. The rotating speed of the turbofan can be selected optionally only by adjusting the power source electric voltage impressed to a direct electric current motor 8 with a switching regulator and the like. Thereby, a heat exchanger 2 can be composed of two units such as a main and auxiliary units, and can be contrived to increase the heat exchanging efficiency, accordingly, the sufficient burning air can be supplied even when the static pressure is increased in a burning chamber 1.