Abstract: A heating system having a primary heat source such as a heat pump, and a supplemental heat source, such as a resistive heater, that is used to complement the heat generated by the primary heat source. Heat is generated by the entire supplemental heat source when the difference between a target temperature and the actual temperature for a space being heated exceeds an extreme predetermined threshold value. Heat is generated by a limited portion of the supplemental heat source when the difference is less than the extreme predetermined threshold value but is greater than another, less extreme, predetermined threshold value. The limited portion of the supplemental heat source is energized and de-energized as needed to generate the amount of heat necessary to achieve a duct temperature setpoint when combined with the heat from the primary heat source.