White Powder on Furnace Vent

[bcawhern1 (Brian Cawhern, CMI)]

This might be it

What is that white powder on furnaces, vents and roof vents?

I finally got an answer from the American Galvanizers Association. Due to inadequate combustion exhaust venting, combustion gases cool before they reach the exterior. This is usually caused by an excessively long- or too many bends in the- combustion vent. Cooling combustion gases condense on the inside of the galvanized steel vent where the condensation reacts with the galvanized steel, and the result is corrosion. The first, kinda liquidy, gooey product of this corrosion is zinc hydroxide, wh…