An instantaneous water heater includes a heating tube, a heating device over a lower end of the heating tube, a spiral pipe held in the heating tube, water inlet and outlet pipes connected to two ends of the spiral pipe, a water-discharge switch installed on the other end of the water outlet pipe; the heating tube is made of metal, and emptied of air, and it contains heat media, which will vaporize into gas rapidly when heated by means of the heating device; the heating device isn't powered usually, yet it will be activated to heat the heat media in the heating tube, which in turn transform into gas to heat water in the spiral pipe, as soon as the water-discharge switch is turned on to discharge hot water through the water outlet pipe.
