Can anyone tell me what this is? First time seeing it.
Looks like corrosion.
What did it taste like?
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It was fine on my eggs
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Any additional photos?
Just a photo of this same substance inside the furnace where it fell thru the holes.
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Carbon monoxide and other un-combusted gases have chemical reactions with the exhaust pipes causing them to rust.
An imbalance of the air fuel mixture can do the same thing.
The stuff falls off the inside of the exhaust pipes and out of the pipe seams and joints.
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Thanks for the info!
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Looks like rust falling off the interior of the flue. I see it all the time.
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The zinc used to galvanize the metal vent pipe reacts with sulfur and other things inherent to natural gas. I’ve seen that dark stuff before… more commonly I’d find white crusty stuff, usually stuck on the joints of the pipe.
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Rust. The acidic water suspended in the flue gases is condensing in the flue and attacking/rusting the galvanized steel. The rust dust falls out. A lot of it indicates that the humidity in the flue gases is not being held in suspension until exhausted outside of the house. You only have a little, so if you didn’t see corrosion on the flue, there probably is not a concern. Even a mild bump on the flue can pepper the top of the furnace. Report the observation and move on.
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Thank you all for the knowledge! Hope to have some input to share soon as I am new to InterNachi and line of work. 
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