Some gasses piggy back on other thermally visible
vapors.This furnace should not have anything rolling out of the flue.The black mist is water vapor which is part of nat combustion.Another part of combustion is carbon monoxide-which is non volitile but deadly .But if you know what to look for and how things are suppose to operate normaly one can determine a conclusion safely.
The point is that the gasfindIR is a great tool but there are things you may see with a lower priced camera that you should not ignor.This was done with an E25 and a cheap camcorder.