Implementation of the Energy Related Products directive in September 2015 had focused attention on the condensing boilers more than ever before since they are the unique boiler technology complying with the requirements of this new regulation. The main benefit of condensing boilers comes from the latent heat recovery by condensing the water vapors from flue gas; the higher the condensation fraction, the higher the energy saving is. The experimental study refers on the heat gained and fuel cost saving by condensing water vapors from flue gas in the case of a heating system with two condensing boilers having nominal outputs of 25 kW and 35 kW, respectively. The maximum condensation fraction of the heating system, determined in real operating conditions, was 0.52, which corresponds to an energy saving rate of roughly 2 MJ/m 3 of fuel. The cost of the energy saved, converted into fuel saving cost, is 22.28 Euro/1000 m 3 for the natural gas price of 426 Euro/1000 m 3 (39.9 Euro/MWh or 0.01108 Euro/MJ) in Ia. i, Romania, February 2020.