The present invention is a method and apparatus for controlling the flow of combustion air and/or combustion products through a furnace which may experience changing flow restrictions. The method provides constant flow through an induced-draft furnace by determining torque values applied to an inducer motor of the furnace from a lookup table, which is established by operating a test furnace under changing flow restrictions, typically by measuring furnace flue gas carbon monoxide concentration. A lookup table according to the invention may include motor operating performance plots for controlling inducer speed under operating conditions, and threshold plots, which may be employed for determining activation of furnace events such as ignition, gas valve energizing, and shutdown. In an adaptive method of the invention, an adaptive lookup table is provided by averaging torque values from each of several lookup tables. A pressure switch is provided having an opening pressure commensurate with a minimum excess air level. The pressure switch has a closing pressure which determines a torque biasing level for the furnace based on motor RPM and torque values determined from the adaptive lookup table.