Abstract: This invention relates to the control of the temperature of heated fluid flow through a so-called single pass electrical heater. Specifically, a controller and accompanying flow circuit including a flow through single pass electrical heater is disclosed which provides for heating of water upon initiation of flow. A two stage control protocol is utilized for routing current under precise control to the heater element. In the first protocol utilized at start up to a control temperature limit slightly below the temperature set point, phase angle of current turn on with zero crossing current turn off control a 220 volt current to the single pass heater element. Once the control temperature is achieved, incremental adjustment of phase angle thereafter occurs with rapid asymptotical approach of the output temperature of the flowing water to a preselected temperature "set point" occurs.