A hanging water heater includes a machine body installed inside with plural quartz heating tubes respectively composed of a quartz external tube having its opposite ends sealed, and a quartz internal tube positioned in the quartz external tube, installed therein with a remote-infrared-ray heating member and having one end connected with an electrode terminal. Each quartz external tube has its lower sidewall bored with water intakes and its upper wall bored with a vapor vent. The machine body is further provided with a water inlet and a water outlet communicating with a circulating tube. The circulating tube communicates with the vapor vent of the quartz external tube, having its wall bored with lots of through holes. The hanging water heater performs heat conduction by vapor, and light and heat radiation, impossible to produce water stains and able to economize energy.