Condensation visible from flue, what is more efficient, a lot or barely visible?

Reed_Richards

I'm old enough to remember the first generation of condensing boilers and they produced much more visible "steam" than their non-condensing predecessors.  If the boiler exhaust is coming out too hot then you won't see any condensation near the flue.  So a lack of visible water vapour is by no mean necessarily a good thing, as some are trying to argue.