A condensing boiler is used as a heating boiler to produce hot water. Compared to conventional boilers, a condensing boiler cools the flue gases as much as possible so that the condensation heat (=latent heat) of the water vapor contained in the flue gas can be used to provide heat. As a result, a condensing boiler achieves a particularly high combustion efficiency. Thanks to the improved efficiency, both the use of fossil fuels and the emission of CO2 and acid-forming gases can be reduced.