There are a few issues that need to be discussed regarding using a slow cooker (Crockpot) on Shabbat
Regarding Shehiya[1]: On the slow cooker there a buttons that allow a person to change the heat settings. There is a concern that a person may come to change the heat settings in a time of need.
Regarding Hatmana[2]: Since the ceramic pot is surrounded by the heating element on all sides and is exposed only at the top
Majority of the people who use the slow cooker on Shabbat, use it in order to keep the food warm for the day meal and rarely for the night meal. The Rama[3] writes that it is permissible to insulate a pot of food on Friday before Shabbat in a material that adds heat[4] so that the food will remain warm for the day meal. According to the Rama, we seemingly resolve our hatmana issue, however another issue arises. The Rama also says that when a pot is only slightly insulated, it is as if it is not insulated at all, which is contradictory to Maran in the Shulchan Aruch who states that even something that is slightly insulated is still considered fully insulated. Therefore, according to Maran the slow cooker would not be permissible to use on Shabbat, because it is considered to be slightly insulated. However, Rav Ovadia Yosef writes[5] that we can permit the use of the slow cooker even according to Maran using the safek sefeka (double doubt) method. The first doubt is that perhaps the halacha follows the opinion of those Rabbis who say that a pot which is only slightly insulated is not considered to be insulated at all, as we mentioned before.
Even if you want to say that the halacha follows Maran, that a pot which is slightly insulated is considered to be fully insulated, we can apply a second doubt which is that perhaps the halacha follows the Rama’s opinion that food which is being insulated on Friday before Shabbat so that it will be warm for the day meal can even be insulated in something that adds heat. Therefore, in a slow cooker in which both these doubts exist, according to Rav Ovadia would be permissible to use on Shabbat.
However, this safek sefeka is difficult to understand because the Rama’s permissibility of insulting for the purpose of the day meal is a combination of two separate opinions, which is a novel idea that is not found by any of the Rishonim[6]. It is also implied by Maran[7] to be stringent even when there are two doubts.
However, there is still room to be lenient with the use of the slow cooker on Shabbat. Simply because there is a small gap of space between the ceramic pot and the heating element. In this instance, even Maran would agree that this is not considered hatmana. Although, since we are unsure whether the gap of space needs to be a large or small gap, it is best to put folded almunimum paper between the ceramic pot and the heating element in order to raise it a little more.
This will also solve our shehiya problem as the heating element will be considered garuf vekatum[8]. Another way to solve our shehiya problem is by turning the heat up to its highest setting since the Rabbis concerns were only regarding raising the heat not lowering it. However, if the person prefers to leave it on a lower heat setting, they should cover the heating element in order to reduce the heat or ensure the food is fully cooked before Shabbat.
Thanks to Yaakov Ganon for translating this question from Hebrew to English.
Footnotes
Shehiya is the act of placing a pot of food before Shabbat on an open flame. The Rabbis were concerned that if they allowed a person to place his food on an open flame before Shabbat, then on Shabbat he will come to check on his food and when he sees that his food is still not fully cooked, he will come to stoke the coals to rush the cooking process. (See Gemara Shabbat 36B)
Hatmana is insulting a pot of food or water either before Shabbat or on Shabbat to keep it warm
Shulchan Aruch 257:1
Also known as davar hamosif hevel. See Mishna Shabbat Chapter 4
Hazon Ovadia Shabbat chelek 1 page 64
Medieval Rabbis between 1040-1400
Shulchan Aruch 257:8
Literally to sweep the coals or cover them with ash