Heat-dissipation units were among the systems of the Constitution-class starships of the 23rd century. They permitted the ship to dispose of excess heat, presumably by transferring it to deep space. They were active systems that required power to operate.
In 2267, Sylvia constructed a model of the USS Enterprise and dangled it in a candle's flame. Using a science she likened to sympathetic magic, she was able to cause the actual Enterprise to heat by mistreating the model in this fashion. Even with the channeling of bypass power into the heat dissipation units, the Enterprise was not able to shed the heat. (TOS: "Catspaw")