When an air heater is used in a boiler:

The use of an air heater in a boiler primarily serves to increase the efficiency of the boiler. Air heaters recover heat from the flue gases that would otherwise be wasted and use it to preheat the combustion air being supplied to the boiler. Preheating the combustion air improves the overall thermal efficiency as it requires less fuel to reach the desired combustion temperature, resulting in reduced fuel consumption and lower emissions. This enhancement in heat exchange effectively maximizes energy use and improves the boiler's overall performance.

In addition, while factors concerning induced draft fans and balanced draft systems are relevant to boiler operation, the focus on efficiency gain through the use of an air heater stands out as the most direct benefit of this configuration.