Air Pre-heaters are basically heat-exchangers installed in the exit flue gas duct of the boiler. The purpose of the air pre-heater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas.
For every degree that the incoming air is heated, that result in less energy required from the boiler. In other words, an air pre-heater saves energy by doing some of the heavy lifting done by the boiler in order to raise the incoming fluid to the appropriate temperature.
