Abstract: A tankless water heater for use in a recreational vehicle uses one or more water chambers, each with a heating element, through which cold water flows and is heated. The system is connected to the alternating current of the recreational vehicle and controls the water temperature by sending the current through one or more TRIACs that change the firing angle of the current in order to increase or decrease power flow to the heating elements depending on the sensed temperature of the outflowing water that is compared against the selected temperature or to match the amount of power flow desired to be sent to the heating elements. The user selects whether to control the unit via a temperature selection or an amount of power flow selection.