An additional high pressure economizer was installed at Unit B1 of the 620 MWe lignite-fired Power Plant “Nikola Tesla B” after 30 years of its operation. An innovative connection of the new additional economizer was applied. It is in parallel connection to the first section of the originally built economizer and it is directly fed with the feedwater from the outlet of the feedwater pump. The analysis of Unit B1 operation with such an economizer arrangement is performed and it is supported by measured data. It is shown that more than 30 MWth of the flue gas waste heat is recovered. The Unit gross efficiency is increased by 0.53 percentage points, which leads to 9.4 MWe of the electric power production. The parallel connection of the additional economizer also leads to the partial feedwater bypass of the high pressure heaters, which enables an increase of the plant electric power by up to 24.5 MWe. The accompanying effects are the reduction of the pressure drop in the feedwater line and the economizers, which leads to the decrease of the energy consumption for the main feedwater pump operation. The applied solution is presented together with measured and calculated energy and economic benefits. •An additional economizer was installed at the steam boiler of an aged power plant.•An innovative parallel connection of new and old economizer sections was applied.•Energy analysis of the retrofitted plant is performed for observed operation.•Energy upgrades depend on the feedwater distribution between economizers' sections.•The unit gross efficiency increases by up to 0.53 percentage points.