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The need to examine combustion devices' combustion characteristics and reduce emissions has gained importance with the implementation of the Energy Related Products Directive in Europe. Therefore, this study aims to investigate flow and combustion characteristics, including emissions in the combustion domain of a condensing boiler. Different premixed gas input velocities and temperatures in the premixed gas burner are considered in the numerical study validated with literature. Effects of inlet velocity and temperature to flow and combustion characteristics are examined parametrically. Variations of velocity and temperature along the combustion domain and CO and NO emissions are obtained at the outlet region. Variations in premixed gas inlet velocity directly influence the residence time and average temperature of the flue gases, therefore, CO and NO emissions. The velocity profiles are effective for flame structure and emission production. In terms of emissions, a substantial escalation occurs in outlet CO emissions with the increase in gas velocity. An increase in NO emissions is observed with the rise in average temperatures and a decrease in NO emissions is observed by reducing residence time. It is concluded that crucial flow and combustion characteristics can be estimated with the use of the current approach to contribute to design improvements and pollutant emission reduction. • Effects of premixed inlet conditions to flow and combustion are examined. • Characteristics of the CO and NO emissions are parametrically investigated. • Premixed gas inlet velocity influences the residence time and flue gas temperature. • Flow and combustion characteristics can be estimated with the current approach. [ABSTRACT FROM AUTHOR]