关键词: 零冷水, 预热, 冷水管, 热水管, 出热水时间

Abstract: ：The paper mainly introduces the first generation technology of full-pipe preheating、the second generation technology of ECO energy-saving preheating and the third generation technology of precise preheating for gas water heaters to implement preheating function. The advantages and disadvantages of the above techniques are also introduced

Key words: Zero cold water, Preheat, Cold water pipe, Hot water pipe, Time of discharging hor water