In this master's thesis is created a conceptual design and installation of an additional heat exchanger in the existing steam generator, in which is needed to reduce wasted heat included in exhaust gases and thus increase efficiency. Based on the thermal calculation of the existing condition, an additional feedwater heater was designed, which would be located at the exhaust gases outlet from the steam generator. The additional heater is designed to fit into the existing space inside steam generator without significant reconstructions on the steam generator itself and also on the boiler room building. The designed feedwater heater is counter-current flow type with a staggered arrangement of pipes in the pipe bundle. The calculation of the additional feedwater heater was additionally verified by computer simulation. Beside additional heat exchanger calculation, a schematic plan of the pipeline distribution from the feedwater pumps to the additional heat exchanger was created and also the necessary fittings for operating and maintaining the additional feedwater heat exchanger were selected. Based on the technical description and requirements for installing this type of heat exchanger, an estimate cost of works and equipment was made.