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A little help please.


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This is a sign of a problem with the furnace exhaust system because the combustion gases are not making it outside before they cool, so they condense and run back down the exhaust pipe, leaving the white residue when it dries. The condensate is corrosive and will damage the equipment.

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Most likely they replaced an old water heater and did not replace the flue pipe. Newer water heaters exhaust is not as hot as the older units, and that LARGE flue pipe is allowing the flue gases to condensate. See it all the time.

I would be willing to bet that the water heater manufacturer does not spec out that large 6" or whatever size it is flue pipe. Looks like a dang fireplace flue pipe![:-bigeyes

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Originally posted by inspecthistoric

Originally posted by gtblum

The heater is 2 years old. Does this white stuff have an official name?

Zinc oxide.

Yup. Rub some on your nose to prevent sunburn. [;)]

The vertical vent is too large for the BTUs in the exhaust from the water heater. The exhaust gas cools too much inside the vent and the water vapor in it condenses. The condensate also pulls NOx and SOx out of the exhaust gas creating a liquid that is a mixture of nitric and sulfuric acid. The acid first dissolves the galvanized coating on the steel vent pipe. The white powder you see, zinc oxide, is what remains after the liquid has evaporated. After the condensate strips the zinc from the pipe, it attacks the steel, eating holes in the vent. The over-sized vent can also affect the draft on the water heater, and you may find back drafting or spillage of exhaust gases at the draft hood.

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The inner liner on a B-vent is aluminum.
Aye, that it is. I was thinking about single wall vent when I wrote my post.

Might the white stuff be aluminum oxide rather than zinc-oxide?
On a B-vent, the acid would attack the aluminum liner first.

I'd think that the condensate shouldn't be in much contact with the galvanized steel.
... until it ate through the inner liner.
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