Thanks for all the helpful advice and suggestions: exactly what I was
looking for. I really like the idea of the greenhouse on the exterior
wall around the fireplace. Just to make sure I understand what you're
suggesting: a door on one side of the fireplace leading to a glassed in
'greenhouse/porch' (perhaps a large window on the other side of the
fireplace to light the room?), with vents moving air from the
greenhouse/porch area into the living room? This sounds very nice.
As for blocking the chimney/fireplace -- didn't quite understand the
suggestion. You covered the opening on the roof with foamboard? May I
ask what L-channel is?

Thanks again for all the help -- very much appreciated!

Art