Single Heat Sink
First, let’s take a look at a Single Heat Sink. It’s a pretty basic piece of equipment that weighs 1 ton and occupies 1 Critical Slot. Each Single Heat Sink equipped on a Mech cools it by 0.13 heat/sec and increases its maximum heat threshold (before emergency shutdown) by 1.3. Single Heat Sinks cannot be mounted with Double Heat Sinks on the same 'Mech.
Double Heat Sink
A Double Heat Sink also weighs 1 ton, but occupies 3 Critical Slots. This means they’re unable to fit into a Mech’s head, center torso or legs. (An exception to this is that clan double heat sinks only take up 2 critical slots therefore they are able to fit in head, CT, and legs.)
Double Heat Sinks come in two varieties, those included in an Engine and those that are added external of engine. Double Heat Sinks included in an engine dissipate .2 heat per second and add 2 to the 'Mechs maximum heat threshold. Double Heat Sinks that are added external of the engine only dissipate .15 heat per second and add 1.5 to the 'Mechs maximum heat threshold. Since engines only include a maximum of 10 pre-installed heat sinks, an engine rating of at least 250 is suggested for anyone looking to maximize their heat efficiency.
The heat threshold of a 'Mech is the amount of heat a 'Mech can build up to before over heating and shutting down. All BattleMechs start with a heat threshold of 30 and is increased by the number of heat sinks you add to your 'Mech. 
Heat Sinks and Engines
Every engine in MWO has a certain number of Heat Sinks built into it. For every 25 engine rating, the engine has 1 fixed Heat Sink inside of it, up to a maximum of 10 (with an engine rating of 250+). The weight of these internal Heat Sinks is automatically built into the weight of the engine.
For every full 25 points of engine rating above 250, one may choose to put an additional Heat Sink inside a Mech’s engine, which will contribute to the total weight of the 'Mech, but won’t use up any of its Critical Slots. Viewing the engine on a 'Mech, it will tell how many of these Heat Sinks can be put inside, and you do so by dragging them from your inventory and onto the Engine Heat Sinks slots.
Switching Between Heat Sink Types
When you upgrade from Standard to Double Heat Sinks, you pay a one-time fee and all of the heat sinks (other than those fixed in the engine) will be stripped from your Mech as you’re likely to want to put on a different number than you had before. You may then purchase and install Double Heat Sinks. In addition, the heat sinks fixed in your engine have their cooling doubled, and you may put Double Heat Sinks in your optional engine slots. Be sure to remember that those extra engine heat sinks are not limited by critical slots, but by a certain amount instead.
When you switch from Double Heat Sinks to Standard Heat Sinks, you’ll also have to pay a one-time fee. This may seem like a downgrade, but you could think of it as; gaining back critical slots at the cost of cooling efficiency. After you switch, all the heat sinks will be stripped from your Mech and you can install Standard Heat Sinks once again.
To switch from one Heat Sink type to another, go to the Upgrades tab in the Mech Lab and choose "Heat Sinks" in the right hand menu. Switching to Double Heat Sinks will cost 1,500,000. Switching to Standard Heat Sinks will cost 750,000.
Effects of Movement Speed
Moving up to 66% of your 'Mechs maximum speed negates the equivalent of one heat sink worth of heat efficiency. Moving any faster than that will negate two heat sinks worth of heat efficiency. Not moving at all will maximize your heat efficiency.