Halacha strictly forbids reheating liquids on Shabbat. Once a soup, gravy or stew has cooled off, one may not place it on a stove (even if it is covered with a “Blech”) or a hotplate to be reheated on Shabbat. Doing so may constitute a Torah violation, which is punishable with Sekila (execution by stoning).
The question arises as to whether one may set an electric hotplate on a timer, and place cold soup on the hotplate before the hotplate is activated by the timer. For example, one may wish to set the timer to activate the hotplate at 10 AM, and place the soup on the hotplate just prior to that time, so that he could have hot soup during Shabbat lunch. Since the hotplate is cold and inactive at the time he places the pot, perhaps we should not consider this person to be reheating the soup on Shabbat.
This issue is subject to a debate among the Halachic authorities. A number of recent Poskim, including the Hazon Ish (Rabbi Abraham Yeshaya Karelitz, 1879-1954), Rabbi Moshe Feinstein (Russia-New York, 1895-1986) and Rabbi Eliezer Waldenberg (Israel, 1915-2006), ruled that this is forbidden, each for different reasons. Hacham Ovadia Yosef, however, in his work Yabia Omer (vol. 10), rules leniently, noting that this situation falls under the category of “Gerama” – an indirect action. Since the person does not actively reheat the soup, but rather indirectly causes it to be reheated, this is permissible. This is also the view of Rabbi Shlomo Zalman Auerbach (Israel, 1910-1995). Hence, one who wishes to be lenient in this regard certainly has authorities on whom to rely.
It must be emphasized, however, that one must be careful not to mistakenly compare different cases, and draw conclusions from one situation to another. We deal here specifically with reheating soup or another liquid that has been fully cooked and then cooled, by placing it on a hotplate before the hotplate is activated by a timer. One cannot necessarily extrapolate from this case to other situations of reheating food on Shabbat.
Summary: The authorities debate the question of whether one may place cold soup or other liquids on an electric hotplate on Shabbat, before a timer activates the hotplate so it can heat the soup. One who wishes to be lenient in this regard certainly has authorities on whom to rely.