The flue gas temperature is 40–75 °C more than that of the steam produced by the steam generators. Hence, before they are discharged, the energy of the hot flue gases can be harnessed by means of a well-designed heat exchanger. A heat exchanger which transfers the energy of the exhaust gases to the water is referred to as an economizer. Economizers with finned tubes allow more heat transfer with less volume.

When the flue gas temperature is decreased by approximately 20 °C, the steam generator’s efficiency increases by 1%.  If the energy of the flue gas is recovered while maintaining its temperature above the condensing temperature, i.e., there is temperature control, the design is termed non-condensing economizer; in contrast, if the flue gas temperature is reduced below the condensing temperature, the design is called condensing economizer.

Owing to flue gas economizers with payback periods of 6–12 months, the efficiency of steam generators can be increased up to 95–98%. The design and production of the economizers are tailored to the steam generator’s capacity, installation method and location.