PURPOSE:To prevent the parts of the water heater other than the heater proper from being overheated at the time of abnormal heating by the heater by a method wherein a heat exchanger is electrically insulated even when the electric insulation property of the heater deteriorates, a leak electric current is controlled to a minimum by using a water supply pipe, a hot water supply pipe and a fixing device all of which are made of electrical insulating material. CONSTITUTION:The heat exchanger 4 is electrically insulated from the other constitutional parts of the water heater such as a chassis 8, the water supply pipe and the like by the fixing device comprising an insulated pipe 19, an insulated stationary plate 18, an insulated fixing washer 19 and a flat washer 20 all made of an electric insulating material and therefore, when the electric insulation of the sheath heater 1 deteriorates, electricity leaking from the sheath heater 1 flows through a human body via water within a heat insulating pipe 17 of the heat exchanger 4, the water supply pipe 2, the hot water supply pipe 3 and an external pipe when the human body touches upon a cock but the human body senses no electricity in using the heater. Further, as the insulating pipe 17 is compact and housed within the body of the water heater, the body itself has two step insulating functions and therefore, the temperature of a functional part such as a flow rate switch and that of the chassis are kept low even at the time of heating so that no trouble or erroneous operation of the water heater takes place and the temperature of the heater fitting wall is kept low to thereby improve safety.
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