I have three circular cast iron vents in my upstairs bedrooms (one in each room) that I can't quite figure out. They seem to be placed where the chimney comes up through the house but Im not sure if they are actually connected somehow. (The fireplace is downstairs.)
It seems strange that the chimney would vent directly into the rooms upstairs. My first thought was to light a fire in the fireplace, and open the vents up but there are cracks in the masonry so I'm a little weary of that. We've never used it.
Well Oilcan looks like you have an older home.
These are most likely old hookups like you guy's said for woodstoves.
You could tell for sure with a small smoke test in the fireplace....or just look up the flue.
It is possible that the vents do not go all the way into the chimney; rather into the cavity around the chimney. That would make them release radiant heat off the chimney into the upper rooms.
It would have been a disaster to vent wood burners into a fireplace chimney. That's a big no no.
BTW Oil Can:
We plumbers use a small smoke bomb (its no bigger than your thumb and has a fuse on it). It doesn't blow up, it just lays there and smokes A LOT. They will usually sustain a lot of smoke for 2 or 3 minutes.