If you want the other type of heat (exp) go here

Heat can be obtained from walking, sacrificing, buying, and fighting. It can be stored in the Erolin Device (small orbs above counter) by clicking the counter. It can also be stored into memory stones by the Fusioneers. Heat is very unstable, so you lose one point of heat every second. The only way to avoid this effect is to store heat in the Erolin Device, or in a Heat Stone.

Heat can be used to multiply the results after battle. The maximum usable heat is 4000, yet the maximum amount you can carry is 4400. This allows for a little buffer so you don't cool down from the maximum effect after a few seconds.