PURPOSE: To make surveillance of quantity of water within a heater duct circuit easy, by installing an air receiver made of a transparent material on an easily watchable location of a warm water drum body for heating through a connecting duct.

CONSTITUTION: An air receiver 12 made of a transparent material is installed in the vicinity of the upper part of a body 3 through a connecting duct 13. Then, an air receiving part within the drum body 3 is prevented from occurring as quantity of hot water within a heater circulation duct 7 can be found easily and only refilling of water in the air receiver 12 is sufficient when the water in the air receiver 12 becomes invisible due to the fact that the water in the air receiver 12 is watchable. Necessity of air ejection can be grasped. In this manner, confirmation of an occurring condition of the air receiver part and the quantity of the hot water within the drum 3 becomes easy.