Using heat for comfort is not a recent innovation, humans have been using fire for thousands of years but we now have a very thorough understanding of the science involved. The chemistry does depend upon the fuel used but all domestic or commercial boilers rely on the combustion triangle.
The Combustion Triangle comprises of the three conditions to create fire, or combustion. If you take any of the three conditions away, the chemical process can not be supported and the combustion stops.
The chemical equation below is representing what is happening with combustion using CH4, Methane (Natural Gas) as a fuel.
Therefore the water vapour that is visible from a Condensing Gas Boiler Flue Terminal is present in all boilers, it is just at a lower temperature and critically, is below the dewpoint of the atmospheric conditions - hence it can be seen.
We must therefore be aware that although pluming at a flue terminal is an indication of low flue temperatures, which with Condensing appliances is desirable, it is still an indication that heat is being wasted. The change of state that is taking place forming condensation is using energy and this is happening outside of the boiler/flue system.
This change of state is using latent heat from our products of combustion and it is energy not being used to heat our property or domestic hot water, which is obviously not good for our boiler efficiency.