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 transfering 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 heat dissipation units operating at maximum power, the Enterprise was not able to shed the heat. (TOS: "Catspaw")