| The argument revolves around the temperature of the heat exchange surface and condensation of the burn products.
Most fuels when burnt produce a number of by product gasses and some of them are somewhat corrosive. If the heat exchange surface does not fall below a certain temperature (because the water behind it is still hottish,) the corrosice gasses do not condense out of the fumes and are passed up the flue and out into the air. However if the surface is cold enough for condensate to form on it then you have a wet corrosive liquid working on the Iron or steel surface which is then heated, so you have hot corrosive materials on it.
In the old days the boiler surfaces were so heavy they would last for years anyway, however today, in order to speed heating the water and for economy in manufacture those sufaces are made as light as possible. In order to squeeze the last therm out of the fuel the more efficient designs also transfer much more of the heat out of the combustion products, sometimes even so much that the flue can be made of plastic. If these boilers and furnaces are not adjusted just right the heat exchange panels are destroyed very quickly, especially if they are allowed to get dusty or dirty. (The particles form nucleii for the corrosive gasses to condense on.) I once saw a a newly installed commercial furnace destroyed in just a year because the temperature gradient was not properly adjusted.
There is a trade off. I know what the risks are but I really have no idea where the value is. In my own house I avoided the problem by having seperate furnace (air temperature) and boiler (hot water) so we can turn the furnace off completely. |