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[Guide] Heat Management

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#1 Amaris the Usurper


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Posted 19 November 2012 - 11:31 AM

Last major update 7 June 2013; lightly updated 23 December 2013 with heat values for new maps. No discussion of multiple-weapon heat penalties ("ghost heat"). Heat Containment is now said to shorten the start-up sequence, but I have not tested this.

The Basics

The basics of heat in MWO are easy to understand. However, keep in mind that the discussion in this section applies only when standing still on a heat neutral map without Pilot Lab unlocks (and not standing in water or getting flamered).

Your mech has a heat level and a heat threshold. Most weapons generate heat when fired, adding to the heat level. At the same time, the heat sinks remove heat from your mech, reducing the heat level continuously until it reaches zero. If the heat level reaches the heat threshold, your mech's fusion reactor will be automatically shutdown, and you will be immobile and unable to fire weapons until it powers back up. This happens automatically when the heat level falls again to the heat threshold. The shutdown mechanism may be overriden for 5 seconds by pressing the O key. Operating with the heat level in excess of the heat threshold incurs risks of ammunition explosion and internal structure damage, as well as voiding your mech's warranty.

The amount of heat generated by each weapon can be found here: http://mwo.smurfy-net.de/. The different weapon behaviors are as follows:
  • Weapons that fire one shot per trigger-pull, such as autocannons and PPCs, instantly generate the indicated amount of heat.
  • Depending on the mech, missile launchers may discharge their missiles in several salvoes. When this happens, the heat is distributed proportionally, according to the number of missiles. For example, an LRM 10 (which generates 4 heat) that fires 2 salvoes of 5 missiles would generate 2 heat per salvo.
  • Lasers distribute their heat evenly over a finite beam duration, ranging from 0.5 to 1 second.
The "heat spike" observed after firing a laser is, therefore, lower than for a hypothetical autocannon or PPC that generates the same amount of heat, since some heat is removed by the heat sinks as it is being generated. The same is true of large missile launchers mounted on hardpoints with few tubes (extreme example: LRM 20 firing from a Narc tube).

The functioning of single heat sinks (SHS) is straightforward. Each removes 1 heat over 10 seconds, i.e., dissipates heat continuously at a rate of 0.1 heat/second. Each SHS weighs 1 ton and takes up 1 critical slot.

Double heat sinks (DHS) are more complicated. Based on the name, one would expect each to remove 2 heat over 10 seconds (0.2 heat/second). However, at present, this is the case only for DHS included with the engine. All other DHS (whether placed in the "engine heat sink" slots or located elsewhere) remove 1.4 heat over 10 seconds (0.14 heat/second). Each DHS weighs 1 ton and takes up 3 critical slots.

The number of included heat sinks is given by

[engine rating]/25,

rounded down, to a maximum of 10, while the number of engine heat sink slots is

([engine rating]-250)/25,

rounded down. Furthermore, a mech must mount at least 10 heat sinks, counting those included with the engine--the remainder must be placed elsewhere on the mech.

For example, provided that DHS are employed, a 205-rated engine has 8 included heat sinks and 0 heat sink slots, while a 355-rated engine has 10 included heat sinks and 4 heat sink slots. The 205-rated engine provides (by itself) 1.6 heat/second of dissipation, and 2 additional heat sinks must be placed outside the engine (to make 10 total). The 355-rated engine provides 2.0 heat/second and has 4 heat sink slots.

Almost universally, it is to your advantage to employ DHS over SHS. The reason is that extra heat dissipation is thereby included without using up tonnage. For example, a 250-rated engine has 10 internal heat sinks. With SHS, the dissipation is 1.0 heat/second; with DHS, it is 2.0 heat/second. It would take 10 tons and 10 critical slots to match the performance of the DHS with SHS.

You should always be conscious of the extra penalty incurred when dropping below an engine rating of 250. Down to this point, decreasing the engine rating only reduces the critical slots available for other equipment, i.e., the heat sinks are moving out of the engine and into other parts of the mech. Below this point, not only do critical slots continue to be lost at the same rate per heat sink, but also more heat sinks (dissipating 0.14 heat/second)--and, hence, tonnage--must be added to make up for the lost dissipation of the engine sinks (0.2 heat/second). It takes roughly 1.43 tons of added DHS to equal the effectiveness of 1 ton of engine DHS.

Effects of Throttle Setting, Environment, and Unlocks

Before proceeding, it is useful to introduce the idea of equivalent heat sinks (EHS). This is my own term, but the idea is an extremely useful one, so bear with me. In a nutshell, this is the effective number of SHS that a given mech has. For a mech with DHS,

[number of EHS] = 2*[number of DHS included]+1.4*[number of DHS added].

Obviously, the rate of heat dissipation (with other effects absent) is then

[dissipation rate] = 0.1*[number of EHS].

Furthermore, the heat threshold is given by

[heat threshold] = 30+[number of EHS].

If throttle, environmental, and Pilot Lab effects are included, these are changed to

[dissipation rate] = 0.1*([number of EHS]*[Coolrun modifier]-2*abs([throttle setting])-[map heat])


[heat threshold] = 30+[number of EHS]*[Heat Containment modifier]-2*abs([throttle setting])-[map heat],

  • [Coolrun modifier] is 1, 1.075, or 1.15, as appropriate;
  • [Heat Containment modifier] is 1, 1.1, or 1.2, as appropriate (see the Comments section below);
  • abs is the absolute value operator;
  • [throttle setting] varies between -1 (full reverse) and 1 (full forward).
The [map heat] values are:
  • Alpine: -5
  • Caustic Valley: +3 (+8 in caldera)
  • Canyon Network: +0
  • Crimson Strait: +0
  • Forest Colony: +0
  • Forest Colony Snow: -1
  • Frozen City: -5
  • Frozen City Night: -5
  • HPG Manifold: -3
  • River City: +0
  • River City Night: +0
  • Terra Therma: +5 (+6 in volcano; heat scale goes to 90% in lava, but dissipation unaffected)
  • Tourmaline Desert: +3
What does the heat gauge really show?

The final task is to explain how the heat gauge on the HUD behaves. The one thing that everyone will have noticed is that reaching 100% on the heat gauge corresponds to shutting down (reaching the heat threshold). But why does the gauge reading
  • hover at a few percent on hot maps,
  • increase when moving, and
  • sometimes not rise above 0% on cold maps when firing weapons or increasing the throttle, even when the rate of heat generation is greater than the available dissipation?
​ Apparently, the percentage scale on the gauge is always calibrated against the heat threshold including Pilot Lab effects only, i.e., 30+[number of EHS]*[Heat Containment modifier], so that a given percentage change always corresponds to the same amount of heat loss or gain. If the heat threshold is lowered (due to throttle or environment), the difference (in percentage points) appears at the bottom of the scale; if it is increased, the extra capacity is included below 0% but cannot be seen. The scale will not rise above 0% until more heat than the increase in threshold is produced. Mathematically, this is given by

[heat percentage] = 100*([heat points carried]+2*abs([throttle setting])+[map heat])/((30+[number of EHS])*[Heat Containment modifier]),

down to a minimum of 0.


The Map Heat values were determined using Testing Grounds. Testing Grounds has been unreliable in some respects (e.g., splash damage), but I haven't observed any discrepancies with live play so far. Let me know if you see anything.

The Coolrun unlock now appears to work as advertised, but Heat Containment may not. For example, firing 6 ER PPCs (66 heat) on Canyon Network (+0) while stationary from a STK-3F with 14 DHS and all Elite unlocks (66.72 predicted threshold) produced shutdown, even though 66.72 > 66. The mech dissipates heat at 2.944 heat/second, and I was able to time a 60% reduction in the heat scale at 12.7 seconds. This amounts to 37.4 heat dissipated (computed from the known dissipation rate, which appears to be reliable), corresponding to 60%. The full scale can be computed as 37.4/0.60 = 62 < 66. This is only a 10% increase (using significant figures) over the base capacity of 55.6; the expected value was 20%.

Fully submerged heat sinks have been stated to function at double efficiency. If not fully submerged, the degree of improvement (up to 100%) is proportional to the degree of submersion. I have not tested this.

Jump jets appear to affect heat in much the same way as the throttle. There is an instantaneous increase in the heat percentage upon firing; the increase is removed gradually once the jets have ceased firing. Presumably, the dissipation rate is also reduced.

Edited by Amaris the Usurper, 23 December 2013 - 08:40 AM.

#2 ElLocoMarko


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Posted 24 April 2013 - 10:57 AM

Do your care to update this?

Map modifiers for Tourmaline and Alpine?
Heat containment seems to be working now.

#3 NinetyProof


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Posted 25 April 2013 - 11:28 AM

Also, You can't "Override" after shutdown starts ... you have to "Override" before shutdown starts.

#4 Amaris the Usurper


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Posted 25 April 2013 - 12:41 PM

View PostElLocoMarko, on 24 April 2013 - 10:57 AM, said:

Do your care to update this?

Map modifiers for Tourmaline and Alpine?
Heat containment seems to be working now.

Now fully updated.

Edited by Amaris the Usurper, 07 June 2013 - 11:26 AM.

#5 Catalina Steiner


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Posted 20 January 2014 - 07:50 PM

I have translated your work. Thanks a lot for your post, it's really educational. Now I understand this set of problems.

Here's the link: Hitze-Ratgeber

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