On 30 November 2016, the European Commission proposed to update the Energy Efficiency Directive, which includes the target to improve energy efficiency by 30 % for 2030. This means that all EU countries will have to find the ways to accomplish the target. The European Commission has proposed to extend the energy saving obligation, which is mandatory till 2020, to 2030. Energy producers and distributers will have to increase their energy efficiency by 1.5 % per year. This statement is obligatory to district heating operators and heat energy producers. One of the ways to accomplish this aim is to effectively use secondary energy (waste energy) in boiler houses. The flue gas condenser is one of the ways how to do it, however, processes, what occurs in condensers are complicated and several parameters influence the efficiency.