Abstract: A heater unit for heating a quantity of water in a reservoir. The flow of water from the reservoir to the heating unit is controlled by thermally responsive means disposed to be exposed to vapors from the water as it is heated by the heating unit. The temperature sensing means comprises a flat bimetallic member for operating a valve to control flow between the reservoir and heating unit. A pan-like heating unit structure is a well in which a spiral heating element is positioned and a sandpipe forms the outlet for the heating unit and operates to back-water over the heating element. A generally horizontal flow channel receives water from the reservoir and leads the water to the top of the well.