The required heat for heating the condensate is taken from the flue gases. The figure below provides an example of how the condensate is heated in the condensate pre-heater.
The heat/steam for the deaerator is extracted from the economizer. In a single-stage pressure system this will be the HP economizer; in a 2-stage or 3-stage system this will be the LP economizer. As a matter of fact, an additional quantity of water is circulated over the economizer, whereby the water heated in the economizer is returned to the deaerator, where it partially flashes into steam. The pressure in the deaerator is regulated by the amount of water to be recirculated.