I had my furnace replaced a few months ago and they moved it about 4 feet from its old location. I just noticed recently that the flue has an opening into the attic. It comes straight down to a "wye" with the angle heading to the furnace flue - but the straight down portion of the wye is just open to the attic space.

Should this be sealed off, or is it some kind of passive draft inducer for the combustion gases. Seems like there should be no opening from the furnace to the outside air.