What exactly is this white powder on the furnace vent? It is combustion by product? There was an opening where a rivet or screw went. Some staining around tape. Didn’t see any signs of rusting or damage elsewhere. 1999 goodman furnace.
This might be it
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Awesome! Thanks a lot Brian
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I once heard it’s a reaction between the zinc on the galvanized steel vent and sulfur which is inherent to natural gas (and increasing due to less pure fuel). This seems to go along with what Brian’s link says.
I have noticed it can be a sign of poor venting if there is an excessive amount. I don’t specifically comment if/when I see it but it can trigger me to start looking more closely at the overall setup when excessive.
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The furnace needs to be serviced and checked with combustion analyzer for proper/clean firing. Too often the gas equipment is left unattended without any service and or inspection for years. 20+ years for a gas equipment is a decent lifespan. Hope the vent had proper rain cap.
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There was indeed a rain cap.
Its zinc oxide likey due to a venting problem. I would definetly write it up as being in need of further evaluation by an HVAC specialist.
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How do I fix the fine white powder coming out of my vents in winter when I turn on gas heater? 8 HVAC companies, a chemical testing lab & numerous handymen cannot figure out problem & how to fix it. Help
Inadequate combustion venting leaves white powder on galvanized steel flues.
InterNACHI Inspection Narrative Library. White Residue on Combustion Vents
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Sounds like a duct cleaning issue rather than something to do with the furnace’s combustion. If combustion gases were in your duct system and coming out of your vents you’d likely dead.
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Agreed. We have too little information. Example, consider new construction and the builder does not put in a filter. Resulting in sheetrock dust being circulated. It might take a minute for that to clear out.
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Not if the gases backdrafting are light. I’ve been in occupied homes with active backdrafting before that has obviously been going yet doesn’t trigger CO detectors and occupants are not dead.
By the time there would be enough exhaust gas in the ductwork to deposit white combustion debris like we see on galvanized vents it would have to be pretty bad. Can we agree on she’d have a really bad headache?
Is that even a thing? 
I have no idea. Maybe. Put some Co detectors up, buy a combustion sniffer. Easy way to find out if exhaust gases are coming in