A system and method for providing a heating system with good thermal efficiency by adjusting combustion air flow in response to fuel-input rate changes over a wide range. An AC motor operates in a slip mode whereby as the fuel-input rate changes, the combustion air density is affected. In response, changes in the load and thus the speed of the motor occur. Consequently, the quantity of combustion air flow increases in response to a fuel-input rate increase. The motor switches to a synchronous speed in response to the fuel-input rate reaching a set value, e.g., 70 percent of maximum rate.