Corrosion above furnace

What would the cause of the corrosion above the furnace? There was also some debris below the flue on top of the cabinet and the flue looked fine with no evidence of corrosion.

There is no central air.

Could it be high humidity. Dirty ducts.

I dont know & need some help.


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Condensation can collect there. The “corrosion” could be airborne particles that remain after the moisture has evaporated. Same with the rust.

Thanks Jae

You could get moisture from a crack in the heat exchanger,How old was the furnace.Matt

12 year old comfort maker. Heat exchanger not visible to inspect. Flame pattern looked good.