Abstract: A unitary electric resistance "in line" tankless water heater for interconnection to an electric power supply conduit, a cold water inlet line and a hot water supply line. The heater includes three separate stacked compartment sections of matching circular configuration and formed of molded nonconducting plastic material. A lower field wiring compartment includes in its side wall the mounting port for interconnection of the power supply conduit and encloses a terminal block interconnected to the power leads of the supply conduit. An upper water heating compartment encloses a circular heating coil element and includes in its top cover member a cold water inlet port and a hot water outlet port for interconnection, respectively, to the cold water inlet line and the hot water supply line. An intermediate compartment encloses a microswitch which is electrically interconnected to the power leads via the terminal block in the lower compartment and to the terminals of the heating coil in the upper compartment.