I have an uninsulated duct running from furnace to the roof. It is much, much warmer than the insulated ducts supplying heat to the house. I can see the heat coming out into the air from this duct. Is this normal!

If this "duct" protrudes through the roof, and that is where you see heat "radiating", then it is a flue pipe used to expel spent combustion gases, much like the exhaust pipe on an automobile.

Yes