Heat is one of the strategic considerations in every engagement. If a 'Mech's heat exceeds the overheat threshold at the end of a turn, it will take internal structure damage. The threshold is 50% + 5% per point of Guts of the 'Mech's heat capacity. Reaching the end of the heat capacity will cause a shutdown and require a turn to reactivate the mech a shutdown is not desirable for your own units, as it gives the enemy a free called shot - of course, it also works in reverse: Overheating enemy mechs is an effective way to get free pot shots at their vulnerable bits.
You can see heat for your own mechs in the user interface - a bar that fills gradually with red, and flashes yellow when you are likely to exceed the heat threshold. In addition, heat affects the mech animations in game (for both your and your opponent's mechs). A mech approaching the heat threshold will appear to be surrounded in heat haze, and look a bit blurry. An overheated mech that is taking damage will glow red-hot.
- Firing weapons. Generally energy weapons cause the most heat and ballistic weapons the least.
- Jumping causes 7.5 heat per jump jet
- Being hit by a Flamer or Inferno Missile. (Multiple flamer and inferno missile hits can be very irritating, and degrade the target's combat capability much more than the damage amount suggests).
- Mechs have an internal heat bank that holds 100 heat. If you gain more than that it shuts down and requires a full turn action to 'RESTART'. Shutting Down also clears all your current evasion.
- Each Mech warrior has their own threshold of heat they can stand in the Mechs internal Bank before it takes internal damage. Their individual Heat Threshold is based on thier Guts skill. Pilots with GUTS level 1-5 have 60. Pilots with GUTS level 6-8 have 75. Pilots with GUTS level 9-10 have 90.
Heat is removed by heat sinks. Every 'Mech gets 10 heat sinks included as part of their reactor. 'Mechs may mount additional heat sinks, which remove 3 heat per turn each . Standing in water doubles the effectiveness of heat sinks, and the biome also has a significant impact on heat sinking.
Exchangers reduce the amount of heat generated when firing weapons by a fixed percentage and are not affected by temperature of the biome or terrain. Multiple heat exchangers are not strictly additive, so it is not possible to add enough heat exchangers to reduce heat to zero. Instead, each subsequent exchanger reduces the percentage of the heat left over after the effect of the previous exchanger, eg: three Exchanger++, which reduce heat by 20% each, would still leave 0.8*0.8*0.8=0.512=51% of the original heat to be dissipated by the regular heat sinks.
Heat Banks increase the overheating threshold and the total heat amount to cause shutdown.