Originally Posted by
GT4
What you posted isn't what he suggested though. The line you posted would only cast it when you have 2 charges (as to avoid wasting charges) which would rarely happen in practice whereas he suggested using it with any amount of charges.
Edit:
The way he described it, this would be his condition for fire blast:
actions.single_target+=/fire_blast,if=(buff.hot_streak.down&buff.heating_up.up)|(!buff.heating_up.up)
(<=> actions.single_target+=/fire_blast,if=buff.hot_streak.down)
Compared to only casting fire blast with heating up active (and optionally with 2 charges, in practice this almost never happens and such only gives like 20 DPS in the sim as well), that results in a loss of 700 DPS with the standard 19M profile in Simulationcraft (as expected). The DPS loss would be even higher for worse gear since you'd have a lower uptime on heating up and a higher chance of wasting the heating up procced by fire blast.
I don't even get how this can be a matter of discussion. Either case gets the same DPS out of fire blast but one maximizes hot streaks procs while the other doesn't. The only reason it doesn't result in a huge DPS loss currently is the very high amount of crit you can currently have in mythic gear.