The Energy-related Products Directive that came into force in all EU countries on September 25, 2015 is focusing attention on the condensing boilers more than ever before since they currently represent the most advanced and environmental friendly heating technology based on fossil fuels. In this context, an experimental study referring to the combustion system of a condensing system of 36 kW nominal heat output - defined in central heating mode and condensing regime - was developed. The analyzed condensing system is of premixed type, consisting of an air blower, a gas valve, a fuel gas-air mixer and a cylindrical multihole burner. Beside nominal heat output, another important requirement is a wide heat output range, for flexibility relative to the load. Consequently, the minimum heat output of the boiler should be as low as possible. This problem of maximum and minimum heat output of the boiler is, in fact, a question of maximum and minimum air flow that enters in the combustion system, which is decided by minimum aria of nozzle of the fuel gas-air mixer. In order to establish the optimum solution for the analyzed combustion system, four fuel gas-air mixers, Siemens type, had been experimented. The paper presents the results of this study.