The present invention features a thermal management system for a hybrid electric vehicle. Such a system includes a heater core supplies heat for heating the vehicle's interior by heat exchange between a coolant and air of the vehicle's interior, a coolant line fluidly coupling an engine and heater core so the coolant being circulated by a coolant pump passes there through. A bypass valve is located in the coolant line in proximity to the engine and is selectively opened and closed according to the on/off state of the engine and so that the coolant does not pass through the engine but passes through only the heater core in the engine off state. Such a system includes a heat exchanger fluidly coupled to another heat source so the coolant is heated by the heat exchanger and supplied to the heater core when in the off state.