Is It Permissible to Use a Slow Cooker (Crock-Pot) on Shabbat?

Greetings, I will try to clarify the matter as succinctly as possible. 

A. Regarding the prohibition of hatmana – according to Ashkenazi custom, in principle it is permitted to use even a slow cooker that covers the entire pot, and this does not constitute hatmana. The main grounds for leniency are: 

1. According to the Rema (Orach Chaim 253:1), even if the pot stands directly over a fire, as long as the fire is exposed from above, it is not considered hatmana and is permitted; and this is the common custom. By analogy, since the pot is exposed from above, it is permitted here as well. 

2. According to the Mishnah Berurah and Sha’ar HaTziyun (Orach Chaim 253:45), it is permitted to place a dish on a blech (metal plate) on Shabbat eve and cover it with cloth, provided there is a gap of air between the pot and the blech. Accordingly, with a slow cooker where there is an air gap between the two pots, it is permitted. However, Rabbi Shlomo Zalman Auerbach, zt"l, forbade this, since this is the usual way to cook during the week, and he considers it actual hatmana of additional cooking, thus not to be lenient even with fully cooked food (Shulchan Shlomo 253:13). Nonetheless, most Ashkenazi poskim hold that in principle leniency is possible, including Rabbi Shmuel Halevi Wosner, zt"l (Shevet Halevi 9:72-73), Rabbi Moshe Feinstein, zt"l, Rabbi Chaim Pinchas Sheinberg, zt"l, and Rabbi Yehoshua Neuwirth, zt"l, as presented in Hilchot Shabbat K'Hilchatah part 14 (pp. 639-645), and distinctions made by Rabbi Moshe Sternbuch, shlit"a (Teshuvot V'Hanhagot 3:106), and Rabbi Yechezkel Rata, shlit"a (Emek HaTeshuvah 4:11 note 2). One who wishes to satisfy all opinions should raise the inner pot so that it protrudes above the edge, which eliminates concern according to all (Shmirat Shabbat K'Hilchatah part 1, ch. 1 note 270, as told by Rabbi Yosef Shalom Elyashiv, zt"l). 

According to Sephardic custom – although the Shulchan Aruch (Orach Chaim 253:1) holds that even slight hatmana is forbidden, Rabbi Ovadia Yosef, zt"l, in his book Chazon Ovadia Shabbat part 1 (pp. 64-65), permits using a slow cooker for cholent eaten during Shabbat morning, based on safek safeka (double doubt), that perhaps the halacha follows those opinions that permit hatmana done for the needs of the next day, and perhaps if it is not covered from above with cloth it is not considered hatmana. He permits this only if the power button is covered. B. All this applies only to hatmana issues. Regarding the laws of shehiya (leaving food on fire from before Shabbat), an electric slow cooker is considered like leaving food on an open flame. Although the heating elements inside are covered by metal, since it cooks in the usual manner, the heat is considered an open flame, and it is forbidden to leave food on it on Shabbat under certain circumstances. If a metal tripod (tess) is placed inside the slow cooker and the pot is set on it, it is considered a covered stove (kli cherom k'toma), and it is permitted to leave food on it from before Shabbat, provided it does not have an automatic temperature regulator (thermostat), as that would be forbidden due to concerns of igniting or extinguishing fire (Mishnah Berurah part 14 p. 646). C. The same applies to returning food to the electric slow cooker on Shabbat in order to heat it. Since this is a usual cooking method, even though the heating elements are not visible, the heat is considered an open flame and it is forbidden to return any food to such a slow cooker on Shabbat, even fully cooked dry food. Only if one places a metal tess inside and sets the pot on it, making it a closed cooking vessel (kli cherom k'toma), is it permitted to return fully cooked dry food to it on Shabbat, provided there is no thermostat. Using aluminum foil inside is not effective alone; only multiple layers of aluminum foil may be sufficient for leniency (ibid. pp. 646-647). 

Blessings, Hillel Meirs