Smell that from the furnace?

Don’t breathe it! It is burnt dust and mites. If you had remodeling, like carpet or new floors, tile, or sanded the floors, the dust from the work has entered into your system.

How? It just works its way down through the cold air side of the furnace. When it goes up through the fan, then the flame cooks it. If you are smelling the dust, that’s how it gets there. You need to have the air duct cleaned.