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4. Placing food on top of a heater or any fire which is covered by a sheet of material:
Based on the above one can say that our ovens which have a [solid] covering [over the flame], even if its [coals] are not swept or covered, as wells as heaters, which [their coals] were not swept or covered, and even if the heater has fire inside it, then [nevertheless] it is permitted to place food on top of it on Shabbos even if that area is Yad Soledes, so long as this food is also allowed to be placed on top of a pot which is insulated on coals. [See Q&A]
The reason for this is because: since the [solid] covering separates between the fire and the pot which contains the food [therefore it is similar to placing food on top of another pot that is on the fire].
Other Opinions: [However] there are opinions which say that even if [the solid covering of the oven] is not Yad Soledes, [nevertheless] one must now place something under the pot in order to separate between the pot and the coals in the oven, in order so this serve as a reminder [of the prohibition to stoke the coals] and thus one will not come to stoke the coals. Now, [although the solid covering of the oven already serves as a separation between the coals and pot, nevertheless] the solid covering [being that it was] already previously made, it does not serve as a [proper] separation or reminder to prevent one from coming to stoke the coals.
The Final Ruling: The latter opinion is the main Halachic opinion, [and thus one must place an additional separation over the solid covering]. [See Q&A]
Summary of placing food by a sheet of metal or earthenware which is sitting over the fire:
Contains all the Chazara restrictions, even if that area is not Yad Soledes, if the coals have not been removed or covered. However, if one places another item on top of that sheet, it is allowed to then place food on top of that item, if the food is dry, or contains liquid which is still warm.
May one initially place food directly on top of a heater/radiator?
Any food which does not have a cooking prohibition [meaning it is fully cooked, and if it contains liquid has not fully cooled down] may be placed on top of a radiator. However food which has a cooking prohibition may not be placed on the heater, even before it is turned on, and if one did so then he must remove the food prior to it turning on.
May one initially place food directly onto a blech?
Areas on the blech that are not directly over the fire: Have the same status as areas which are directly over the fire.
If the area can heat the food to Yad Soledes then it is forbidden to initially place food on it. However in a time of need one may be lenient to return a pot to the belch even if he placed the pot down and let go of it, and is thus missing one of the conditions of Chazara. However as already explained it is best to place an upside down pot on the blech and then place the food on top of it.
If the entire area of the blech cannot heat the food to Yad Soledes: In such a scenario it is permitted to place [even uncooked] food on it.
If parts of the blech can heat the food to Yad Soledes while other parts cannot: In this scenario there is a dispute amongst the codifiers if one may initially place food on the non-Yad Soledes areas. Practically one may be lenient when in addition to this there joins in other leniencies. [Such as it was originally on the blech but was removed without intention etc.]
May one place Chalas on the blech in order to warm?
Rav Sholomo Zalman Aurbach writes that it is allowed to place chalahs to warm on areas of the blech which are not directly over the fire, being that this is not the regular way of baking during the weekday. However, Rav Farkash rules that one should not be lenient unless there is a great need to do so.
May one initially place food directly on an electric plate?
By an electric plate with more than one setting it is forbidden. If there is only one setting it is disputed amongst Poskim as to the ruling. Practically by a time of need seemingly one may be lenient by an electric plate that only has one setting. Nevertheless, as explained previously it is best to place the food on an upside down pot, in which case it is permitted according to all opinions.
May one move food on a Blech from one place to another place?
See above Chapter 1 Q&A
 Admur 253:26; M”A 253:31
 Halacha 3 in which Admur ruled that a food may be placed on top of a pot.
 Such as a solid metal sheet [blech], or a solid sheet of earthenware. However see Q&A 2 regarding a blech.
 Which as well have a covering over their flame
 Meaning not just coals.
 Meaning so long as it is permitted to place a food on top of another pot which is on the fire, similarly it may be placed on top of the metal sheet which covers these ovens. However when prohibited to be placed on top of another pot, such as if the food contains liquid which has fully cooled down, then similarly it would be prohibited to place it over the metal sheet which covers the oven.
 So rules Admur also in 253:22 like the stringent opinion: “For this reason it is proper to be careful to not remove the pot off from the Kirah or the Tanur which it was cooked in and place it [On Erev Shabbos] close to sunset, on a heater, which is not swept or covered, if it that area is Yad Soledes there. [This however is] with exception to if one places something under the pot, just like one is required to do on Shabbos itself, as will be explained [in Halacha 26].”
 Shabbos Kehalacha pages 409-410; Piskeiy Teshuvos 253:28; Igros Moshe 4:74-34; Bris Olam Amira Lenachri 16-8
 As this is not the usual way of cooking and thus does not appear like one is initially cooking the food on Shabbos. So rules the Igros Moshe and the Sheivet Halevy. Now although in 253:26 the Alter Rebbe seems to rule clearly that this is forbidden, and that one must place another vessel on top of the heater, Rav Farkash explains that the Alter Rebbe was only referring to a type of heater that although was not used to cook food, but was used to heat up food, while today our heaters since they are not used at all to heat up foods, therefore it is allowed to place fully cooked food even directly on them.
 Shabbos Kehalacha Vol. 1 page 411-414; Piskeiy Teshuvos 253:28-29 and footnote 264; SSH”K 1; Igros Moshe 1:93; 4:74-32
Other Poskim: Some Poskim are lenient to permit heating all foods on a Blech. [Zera Emes; Mahram Brisk 2:76; Maharam Shick 117; See Shabbos Kehalacha 10 footnote 23 p. 412; See Piskeiy Teshuvos 253 footnote 264 in name of Degul Merivava]
 See Piskeiy Teshuvos 253:29
 Based on 253:26. So rules the Igros Moshe and Shemiras Shabbos Kihilchasa and other Poskim today.
 This leniency is allowed being that there are many Poskim which even initially allow one to place fully cooked food on a blech, as they do not consider this similar to Halacha 26 being that here the blech was explicitly placed with intent to cover the fire, and thus according to all there is no suspicion that one may come to higher the flame.
 Shabbos Kehalacha ibid based on 253:21; Igros Moshe 4:74
Background: It is permitted as it is exactly similar to a Kirah oven which has a covered fire in which we rule that one may place even uncooked food on top of it in an area which is not Yad Soledes. However, Tzaruch Iyun as the stringent opinion [mentioned in 253:26 or Halacha 4 above] rules that even if the top of the heater is not Yad Soledes one may not place it there directly without first placing another separation between it. However, perhaps here by a blech which is not Yad Soledes there is more room to be lenient, as some allow even initially placing a cooked food on a Blech being it is not similar to a heater of back in the day. Hence certainly one should be allowed to be lenient by an area which is not Yad Soledes. Vetzaruch Iyun as perhaps the same way we are stringent on a Blech by an area that is Yad Soledes, perhaps we are also stringent by an area which is not Yad Soledes, as the same suspicion applies in both areas [according to the opinion which compares Blechs to heaters of back then]-perhaps one will come to stoke the coals! Practically Rav Farkash rules leniently in this matter and so rules Igros Moshe 4:74.
 See Piskeiy Teshuvos 253:29
 Shabbos Kehalacha Vol. 1 page 414-415; Piskeiy Teshuvos 253:27
 So rules also Piskeiy Teshuvos ibid to be stringent.
 Shabbos Kehalacha Vol. 1 pages 416-418; Piskeiy Teshuvos 253:28
 The question here is can an electric plate be viewed similar to a sheet of solid material that is on a fire, which is discussed in Halacha 26, in which case it is permitted if there is only one electric setting being that it has no suspicion that one may come to higher the flame, or do we view it like a regular fire being that it is common to use the plate to heat up food.
The Sheivet Halevy rules that it has the same laws as does a blech, which one may not initially place food on, being that it is common to heat up food on the electric plate.
However, the Igros Moshe rules that if there is no variations of heat on the electric plate then it is permitted to initially place food directly on it. Rav Shlomo Zalman Aurbach rules similarly that it is allowed, based on that the entire problem with a fire covered with a sheet of solid material [above in Halacha 26] is because one may come to stoke the coals, and thus here since there are no variations of settings, this suspicion is not relevant and thus it is allowed.
The question here is can an electric plate be viewed similar to a sheet of solid material that is on a fire, which is discussed in Halacha 26, in which case it is permitted if there is only one electric setting being that it has no suspicion that one may come to higher the flame, or do we view it like a regular fire being that it is common to use the plate to heat up food.