Ergo, you'd cast your Fblast on cooldown at 100% crit (which will never happen, but for the sake of argument). Below 100% crit, you save your charges for a Heating Up. You don't 'just cast' Fblast because you've had a 5 hit crit-chain and are sitting on 2 charges. What if the 6th doesn't crit? Also, Blast Furnace is negligible, especially if you weigh it up against losing a potential instant Pyro.