If a pot containing food was in a hot oven, and the pot was removed from the oven so that food could be removed from it, it may not be returned to the oven. There are two reasons for this. First, since there is nothing reducing the temperature of the oven, there is a concern that when one returns food to the oven, he will forget that it is Shabbat, and turn up the temperature. Second, this action resembles cooking. One possible solution to both these problems is to place the food within a four-sided metal box inside the oven. In such a situation we are not worried that one will raise the oven’s temperature, nor does it resemble cooking when he returns the food to it, since normally people do not cook food in a box. It would seem that one may also be lenient if one puts a pan or overturned plate on the bottom of the oven, and then inserts the food. Additionally, one should cover the oven knobs.
In order to return food to an oven, one must meet the conditions necessary to permit haĥzara outlined above (section 19). In other words, since food was in the oven, and the person taking it out intended to return it, this does not resemble cooking. However, one may not place cold food in a hot oven, since that does resemble cooking.
If the oven is controlled by a timer, one may place fully cooked food in the cold oven, so that it will heat when the oven goes on. This does not resemble cooking, since the oven is off. In order to avoid the concern that one might raise the temperature, the knobs must be covered or the food must be placed on an upside-down pan. However, some poskim forbid this because they maintain that even though the oven is off, putting food in it to heat resembles cooking. However, in practice, since the law is rabbinic, one may rely on the lenient opinion.
Some people have a plata with a compartment that is set up on top of it, into which pots can be placed. Food that was removed from such a plata may be returned to the compartment. This does not resemble cooking because no one cooks in such a compartment, and its sole purpose is to keep foods on the plata warm (SSK 1:79). Those who place fully cooked cold food on the plata (as explained in section 18) may put food on this kind of plata as well. Those who are stringent may place the food on an inverted plate. Placing things into the compartment is not considered hatmana (see below), because the prohibition of hatmana is relevant only when the pot is in full contact with what surrounds it, while in this case not all the sides of the pot touch the compartment.
. One may not return a cooked food to the oven for two reasons: lest he stoke the coals (increase the flame), and because it resembles cooking. So state SSK 1:19 and Minĥat Yitzĥak 3:28. However, if one places a box inside the oven and replaces the food inside the box, according to Igrot Moshe OĤ 4:74, Bishul 27, it is permitted, because once one takes action to minimize the heat, there is no fear that he will forget about Shabbat and increase the heat. It also no longer resembles cooking. Hilkhot Shabbat Be-Shabbat 1:5:25, p. 237 states similarly in the name of R. Elyashiv. Shabbat Ke-halakha 1:9:36 permits if one returns the food on top of an inverted pan, thereby minimizing the heat (the equivalent of covering the coals) and demonstrating that he is not cooking. According to AHS 253:17, nowadays there is never a concern that returning food to an oven resembles cooking. Earlier sources explain that people used to cook in an oven and heat food on top of it. Therefore, when anyone placed food into an oven, it resembled cooking. But today’s ovens are built in such a way that one cannot heat food on top of them, and thus even heating food (without cooking) is accomplished by placing food inside the oven. Therefore, putting food in the oven does not necessarily indicate that one is cooking. Shevet Ha-Levi 3:48 and Menuĥat Ahava 1:3:8 state this as well. Nevertheless, it is still necessary to take some action that corresponds to covering the coals to remind people not to turn up the flame. According to Shevet Ha-Levi, one does this by placing a pan as a barrier between the heating element at the bottom of the oven and the food, while according to Menuĥat Ahava this can be accomplished by covering the knobs. Yalkut Yosef 253:8 permits returning the food even without an upside-down pan or covered knobs, because in its opinion, heating elements that are not visible are considered covered, and there is no concern that one will adjust the flame. All this is on the condition that one meets the requirements necessary to permit haĥzara.. R. Levi Yitzhak Halperin, in his book Kashrut Ve-Shabbat Ba-mitbaĥ Ha-moderni, permits this because when the oven is off, placing food inside it does not resemble cooking. In order to prevent the possibility of raising the temperature, the knobs should be covered or removed. It is also preferable le-khatĥila to set the oven at a temperature lower than is generally used for cooking. This is cited in Peninei Hora’ah, p. 113 and is the ruling of R. Lior. Yabi’a Omer 10:26 is even more lenient, in a sense. Ĥazon Ish 38:2 prohibits this, as do Minĥat Yitzĥak 4:26:10; SSK 1:32; Orĥot Shabbat 2:68; Hilkhot Shabbat Be-Shabbat vol. 1, p. 251; and Shvut Yitzĥak 2:9:1-2. Also see Harĥavot on section 17.