Thanks to both of you. I strongly suspected that the old time builders used mostly rule of thumb, or it worked last time, lets try it again. Most of them weren't really engineers in the modern sense of the term, just solid practical mechanics of a type that almost doesn't exist anymore. Before there were bridges across 'most every river the only limit on chimney height was what the owner wanted. Sitll, I would guess that you could get them too short or too tall. I was hoping that some of the old rule of thumb ratios were still known. Just for curiosity does anyone know the ratio of flue area to stack area on Julia Belle Swain or Natchez?