Dear SB:
Just to make sure, you are not trying to recycle flue gasses, correct?
I can give you a handful of search terms that will get you close. Also, cross posting to sci.engr.mech.
"natural convection" "Grashof number" "Rayleigh number" "Nusselt number"
The first term is the name of the process that produces what you are trying to recycle. The other three are dimensionless groupings of dimensioned quantities important to natural convection. There is in general NO closed form solution... only the ability to scale an experimental model to real life via "empirical relations".
Perhaps others can be more helpful. I know there are products available that will blow air in a duct constructed *around* the flue, to capture the extra heat, and blow it into a room. Capturing hot air from a less well defined area will be more difficult. And I'd start with 1000 scfm, and go from there.
David A. Smith