Just did the most thorough tests I could think of with heat exchangers. They will accept heat from any source (directly from fuel cells, from heat outlets and from heated components such as coolant cells and capacitors), but will only give that heat to heat inlets and other heat exchangers. They will NOT give heat to heat vents, coolant cells, or even capacitors (tested even though heating a capacitor is always a bad idea).
3,315,783 ft. with the tank, golden gun and 2 (out of a stockpile of 12) nukes. Admittedly, I got very lucky and grabbed 3 ammo packs while flying at mach 30! Still, 3 million is not that hard to hit.
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