Abstract: A heating system has a primary heat source such as a heat pump, with a capacity that occasionally needs to be supplemented from a supplemental heat source such as an electrical resistance heater. The heating system heats and circulates a medium such as air via supply and return ducts. A thermostat responsive to the heated space has two outputs for operation at different user-selected temperatures defining stages of operation at which the primary and secondary heat sources are normally drawn upon. The invention controls the duct temperature by activating the supplemental heat source according to a further setpoint, that is adjusted by a processor or other control circuit. A temperature sensor allows the control circuit to determine the duct temperature. The control circuit is also coupled to the two thermostat outputs and to a memory register storing the variable duct temperature setpoint.