I have a 2 year old Goodman gas furnace model CAPF1824B6CA with two 3/4" female thread openings next to each other above burner in bonnet. I know the 3/4
Opening on right is a/c conditsation line, what is the one to the left of conditsate line, when heat is on heat comes out of this opening, should it have a plug in it or be left open.