הלכה על שבת 4:1

He notes that in most refrigerators, metal is not heated until it glows. He explains that even though there are gases that are heated in the refrigeration cycle, heating these gases does not constitute an act of bishul (cooking). Among his reasons are that the gases are not heated by a fire source (see Rambam's Commentary to the Mishnah, Shabbat 4:1) and that heating a gaseous substance does not constitute bishul.