The combustion of natural gas produces water vapor and carbon dioxide etc.  One cubic meter natural gas produces approximately 1.5kgs of water vapor (appx 2M³). The excess air coefficient of the condensing boiler is between 1.2 and 1.3, the dew point temperature is about 54°C, and the general exhaust temperature is about 60°C. When the smoke comes out, it is already in the state of small water droplets, not steam. The white smoke seemed is actually tiny water droplets. Therefore,it is already white smoke before contacting with cold air (in winter) to condense.

The exhaust smoke temperature of non-condensing boiler is above 130°C and it comes out of the flue invisible.When it further spreads out, it will be condensed into visible white smoke (tiny water droplets) by cold air (winter). It therefore appears to have less white smoke than condensing boilers. Some non-condensing combi boiler with higher exhaust smoke temperature, or larger fan power (the flue gas is flushed farther), have less white smoke.