A proper understanding of the Talmudic laws of cooking on Shabbos requires the translation of ancient kitchen technology to modern appliances. This is certainly the domain of the greatest halakhic authorities, aided by technology experts. But no one lives forever and even great communicators can sometimes be unclear. What happens when a great rabbi’s family disputes his students’ interpretation of his ruling? Who owns his legacy?
Talmudic ovens come in three sizes, and the rules differ slightly between them. Nearly all authorities agree that our stoves, when the top is covered with a metal sheet (blekh–and preferably the knobs are also covered), are categorized as a kirah that is gerufah and ketumah. This means that despite the lack of concern of stoking the fire, because the fire (and knobs) are covered, you still may not put cold dry food on the blekh because you look like you are cooking (michzei ki-mvashel). [While R. Ovadiah Yosef (Yechaveh Da’as 2:45) and R. Nachum Rabinowitz (Melumdei Milchamah, no. 50) reject even the concern that you look like you are cooking, they are far in the minority.]
Less similar to ancient devices is the electric hot plate. Some have only setting, below cooking temperature. Other models have a sole setting above cooking temperature and others multiple settings. Are they considered the same as stovetops or, since they are only used for warming pre-cooked food, are they treated more leniently? The Tzitz Eliezer (8:26:5) argued that hot plates are equivalent to stovetops. R. Shimon Eider asked this question to R. Moshe Feinstein, who replied that they are different; hot plates are used for warming and retain no concern that you appear to be cooking on them (Iggeros Moshe, Orach Chaim 4:74:35). Therefore, if an electric hot plate has only one setting, so there is no concern you will turn its knob, then you may place cold dry food on it on Shabbos.
However, R. Feinstein’s answer is not entirely clear. He responded to a question regarding electric hot plates in general (with only one setting) by discussing one that cannot reach cooking temperature. Does he permit all electric hot plates with only one setting? R. Eider, to whom this responsum was addressed, understood R. Feinstein that way (The Laws of Shabbos, n. 566). R. Simcha Bunim Cohen (The Shabbos Kitchen, p. 43) also rules that you may place cold dry food on an electric hot plate with only one setting based on R. Feinstein’s responsum. R. Eliezer Melamed (Peninei Halakhah, Shabbos 10:18 n. 19) quotes a range of opinions on the subject, attributing a lenient view to R. Feinstein based on his responsum.
However, R. Mordechai Willig (Am Mordechai, Shabbos 7:2) states that R. Reuven Feinstein told him that his father only permitted using in this fashion a hot plate that does not reach cooking temperature. I have heard the same in R. David Feinstein’s name. R. Feinstein’s (the father) responsum is difficult, facing problems according to both readings, as R. Willig explains (if the hot plate does not reach cooking temperature, why the need for only one setting?). Who knew better what R. Feinstein said on this subject–his sons or his students? I don’t claim to know and this is only one of a number of cases where such a dispute exists.
Regardless, R. Willig argues, even without R. Feinstein’s support, that because people do not cook on an electric hot plate, you may put cold dry food on one on Shabbos even if it reaches cooking temperature (as long as it only has one setting). R. Eliezer Melamed (ibid.) quotes R. Dov Lior as ruling similarly although, of course, other authorities disagree.[As always, ask your rabbi before acting on any halakhic issues you read online or elsewhere.]