SUPPLEMENTAL HEAT CONTROL SYSTEM

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. The duct temperature is compared to the duct temperature setpoint and the supplemental heating means is enabled to increase the duct temperature when the duct temperature falls below the variable setpoint. The control circuit adjusts the duct temperature setpoint upwardly upon occurrence of the temperature in the heated space dropping below a lower one of the space temperature setpoints (indicating that the heat pump is not keeping up), and downwardly upon the temperature in the heated space exceeding a higher one of the two space temperature setpoints (indicating that the heating capacity is adequate and use of the resistance heater should be reduced).