A thermal management system includes a coolant pump, high-voltage electric heater (HEH) for heating the coolant, a heater core, a blower directing air to the heater core, a cabin heater valve (CHV), sensors, and a controller. The CHV has an Engine Bypass position blocking coolant flow from the HEH into the engine, and an Engine Link position directing coolant from the HEH into the engine. In a method, the sensors measure engine outlet coolant temperature (ECT), inlet coolant temperature (ICT) to the HEH, inlet air temperature into the heater core, and outlet air temperature from the heater core. The controller calculates a target coolant temperature (TCT) value as a function of the air temperatures and mass flow rates, and controls the CHV via position control signals such that the CHV is switched between the Engine Link position and the Engine Bypass position when ICT equals the calculated TCT value.