PURPOSE:To continue whirling stream, by a method wherein the end of a combustion air passageway is not throttled and is provided with the same diameter as the root thereof so as not to regulate air stream. CONSTITUTION:Combustion air from ventilating fan 6 is introduced into an air passageway 4 and fuel from a fuel pump 17 is injected through a nozzle 14, the fuel is ignited by the arc of an electrode 19 and is burnt in a combustion chamber 3 whereby water in a boiler body 1 is heated. Air stream, which passes through the slits 11 of an air passageway 4, becomes a whirling stream about the center of a cap 7 while a part of air stream, passing through an air flowing port 8, is rolled into the outer peripheral side of the cap 7. There is no gap between the outer periphery of the cap 7 and the internal peripheral surface of the air passageway 4, therefore, the distribution of secondary air will never be changed due to the variety of the gap whereby a flame keeping ring is made uniform and a flame keeping force may be increased.
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