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outside air
The furnace does draw in outside air through a fresh air vent however, this air is sent thru the furnace intake filter system before being heated / cooled... the filters are the normal furnace filters that are replaced monthly or as needed.... Since the furnace filters are never really all that dirty, I have to assume that the dust is coming in thru something else... I have sealed the cavity of the furnace to make sure everything that enters it goes thru the filter....still dust
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