A water boiler comprises an inner tank, an electric heating pipe, and a controller. A lower cover is attached to a mouth of the inner tank. The lower cover comprises an inlet pipe and an outlet pipe, wherein an outlet port at a lower part of the inlet pipe is in communication with lower part within the inner tank. An inlet port of a lower part of the outlet pipe is in communication with an upper part of the inner tank. The lower cover is arranged beneath the upper cover. An upper part of the inlet pipe passes through the upper cover. A cavity is formed between the lower cover and the upper cover. The cavity is partitioned into an upper chamber and a lower chamber by a diaphragm. The lower chamber is in communication with an outer space.