Due to increasingly strict requirements for environmental protection and energy efficiency, defined by the laws of the Republic of Croatia and European directives, which include the reduction or elimination of the use of fossil fuels for energy production, thermal power plants are facing serious targets for more rational heat and electricity production. One of those thermal power plants is also the thermal power plant EL-TO Zagreb. By stopping the use of heavy fuel oil as operating fuel, the possibility of additional utilization of waste heat from flue gases created from natural gas combustion was opened because those flue gases can be cooled to a lower temperature than it was the case with flue gases from fuel oil combustion. EL-TO Zagreb is one of the oldest thermal power plants in Croatia and, despite the fact it is currently going through a huge investment cycle which includes building of new, more efficient production units, it still takes care of achieving energy savings and increasing the level of operation of old production units. One of the proofs that energy savings are achieved not only by purchasing new devices, but also by renewing the existing ones, is the installation of additional district water heaters into the flue gas ducts of the plant’s production units. Additional district water heaters were installed into the common flue gas duct of the steam boiler K6 and two hot water boilers VK3 and VK4, into the flue gas duct of the steam boiler K9 and into the exit of heat recovery steam generators UT1 and UT2. This final thesis describes the technical part of the installation of district water heaters and the achieved thermal power of each heater after installation, it gives an overview of legislative frameworks in terms of energy savings within HEP Group and the contribution of installation of district water heaters, as a measure of savings in the energy conversion sector, to achieving the target of energy savings on an annual basis and in the cumulative period. An economic-financial analysis was performed by using the following financial decision-making methods: payback period, net present value and internal rate of return. Also, the sensitivity of financial criteria to changes in key variables was examined for each installed district water heater separately.