A gas-fired air heating furnace has a modulatable supply air blower and a modulatable gas valve operatively connected to the combustion air heat exchanger burner. A calibration sequence of a microprocessor-based control system is utilized to automatically determine the precise relationship between the gas valve setting and the actual heat transferred by the heat exchanger to air flowing through the furnace by measuring the actual air temperature rise across the heat exchanger obtained using initial calibration settings of the blower and gas valve. The microprocessor uses the results of its calibration sequence to establish an adjustment correlation between the blower and gas valve settings which the microprocessor subsequently utilizes to adjust the flow rates of the blower and the gas valve in a manner maintaining a predetermined, generally constant air temperature rise across the heat exchanger while the furnace alters its overall air heating output rate in response to changing heating demands from the conditioned space served by the furnace.