The Erolin device (small orb above the counter) can be used to store heat. Initially there are 2 heat orbs. Each orb stores 1000 heat, and can be activated by clicking the counter. Then, each orb may be clicked to unlock it. The maximum ammount of heat you can hold is 4400, but amounts beyond 4000 grant the same effect as 4000 in combat.
You can also buy 4 additional Erolin Devices from the MD shop , These items do not come back when you reset the shop.