The invention discloses a boiler, comprising a water inlet, a water reservoir and a water outlet, wherein an electromagnetic valve is arranged between the water inlet and the water reservoir; the water reservoir is provided with a control device for controlling the motion of the electromagnetic valve; a buffer room is arranged between the water inlet and the electromagnetic valve; a filter screenis arranged in a conduit between the electromagnetic valve and the water reservoir; the control device comprises a water level switch being connected with and controlling the electromagnetic valve; apushing mechanism is arranged below the water level switch; and the pushing mechanism is contacted with the water level switch for closing the electromagnetic valve. The boiler can reliably keep the water level of the water reservoir to be unchangeable; a floating body floats on the water surface of the water reservoir, and the push rod on the floating body always lies opposite to the water levelswitch; when the push rod floats upwards to contact the water level switch, the electromagnetic valve is closed and water does not enter any more, thus leading the water level of the water reservoir to be maintained at the preset position; synchronously the filter screen can reduce the scale deposit quantity of the water reservoir and the conduit is not heated, thus ensuring the drinking water sanitation maximatily.