The condensing gas boiler technology has received increasing attention due to its very high efficiency of more than 90%, which is an increase of more than 15% compared to non-condensing boiler devices. With this, condensing gas boilers offer the chance to decrease the energy consumption and CO2 emissions for domestic hot water and heating. While further increasing the energy efficiency of condensing boiler devices, pollutant emissions have to be considered as well since they are a threat to human health and therefore subject to continuously intensified governmental restrictions. In this study, a comprehensive investigation of the full-scale heating unit of a commercial condensing gas boiler was performed. Local measurements at different axial positions revealed an inhomogeneous distribution of CO and NO emissions due to varying temperature levels in the burnt region. Resolved simulations with finite rate chemistry identified a quenching of the CO oxidation reactions due to a fast depletion of OH radicals. Consequently, the CO concentration in the cooled exhaust gas is significantly higher than expected from chemical equilibrium calculations. Regarding NO formation, the majority of NO is found to be produced within the flame front, while only a small part is formed in the postflame region. A detailed pathway analysis pointed out that besides the well known thermal NO pathway, the NNH pathway has the highest contribution to the overall NO emissions. These insights open up possibilities to develop new condensing boiler generations where emission levels below the permitted limits can be achieved.