Concerning the heat indicator, on the bottom of the UI, next to the health bar. I always thought that when the fire ball filled the whole circle my character would "Awaken" or be "On Fire", a state in which the character deals extra damage as a counter measure to the massive power and ability demonstrated by end of level bosses. Apparently Spiral Knights does not yet have this feature.

It makes sense, when the player runs through the level collecting heat. I mean like in basket ball when you score so many shots, people say that you are on fire. Like so when players collects so much heat, they should be able to activate a mode in which their character goes "on fire".

-Apsalus

P.S.
I used the awakening feature in Elsword as a reference.