Using the latest forge (10.13.2.1240) and Botania 1.3.140.

I have a floating exoflame above a mana pool (with enough mana in it) and 6 furnaces beside it. The exoflame only heats 1 furnace at a time. Seems to be the furnace at the lowest X,Y,Z coordinate that gets heated up if it has something to smelt/cook in it. Once nothing is left to cook in that one, the next in line gets heated up.