Abstract: The feedwater heater of this invention comprises a plurality of heating tubes arranged in U form, with feedwater being passed therethrough, a feedwater inlet chamber having a feedwater inlet for introducing feedwater thereinto and a tube plate assembling the ends of said heating tubes and also having formed therein a feedwater reservoir, a feedwater outlet chamber having a feedwater outlet for discharging the heated feedwater and a tube plate assembling the other ends of said heating tubes and also having formed therein a feedwater reservoir, a first cylindrical body or shell having disposed therein a tube nest consisting of the U-formed heating tube portions positioned on one side as well as support plates holding said heating tubes in position, said cylindrical body having its one end communicated with said feedwater outlet chamber and also provided with a hot steam inlet for introducing hot steam that serves as the feedwater heating source, a second cylndrical body or shell having disposed therein a tube n