That link was a pointless argument as the flue in question was 11 years old at most.
Yet the first post says it is a 15 year old boiler.
It won't remain as is though. It's being converted to a fanned, condensing flue.
The vent is there, agree that the new boiler will create a visible plume, but if there is an easement to vent then changing the appearance of the material vented will not affect the easement.