H. Oven or heat source is less than Yad Soledes:

The definition of Yad Soledes Bo:[1] A Yad Soledes area is defined as an area that is hot enough that if the food that he is returning there were to be cold, it would be able to heat it up to the point of Yad Soledes [110 Far.].

Placing by side:[2] It is permitted to place food of any type, even if not cooked at all, to the side of any heat source that is not Yad Soledes.

Placing on top of stove, Blech, or electric plate:[3] Placing food on top of a stove top is permitted if the stove top is less than Yad Soledes, and the fire on the stove top is no longer on. [Thus, if one turned off the fire before Shabbos, it is permitted to place food onto the Blech on Shabbos if it is no longer Yad Soledes.] If the fire is on, then it is disputed if one may only place food in a less than Yad Soledes area even if the fire is covered. [Thus, it is disputed if one may place food on top of a Blech or electric plate in an area that is not Yad Soledes.] If the entire stovetop, Blech, electric plate area is not Yad Soledes, despite the fact that the flame is on, then it is permitted to place food on it.[4]

Placing in oven:[5] Placing food inside an oven is never allowed under any circumstances, even if the oven is less than Yad Soledes. [The above applies regarding ovens of medieval times, however, regarding placing a food into a modern oven which is not Yad Soledes, seemingly this would be subject to the same dispute as mentioned in Halacha 4C regarding if fulfilling the Chazara conditions allow one to return food into an oven.[6] Practically, however, since even by the ovens of back then there are those who are lenient, seemingly one may be lenient to return food in a modern oven which is no longer Yad Soledes. Vetzaruch Iyun if one would be allowed to even initially place food there, even uncooked food.[7]]

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[1] Admur 253:21

[2] Admur 253:21 “All the above restrictions [of returning a food near an oven] is only when one is returning the food on Shabbos to an area which is Yad Soledes. However, if it is not Yad Soledes in that area, then according to all opinions one can place food near the oven, even [near] the Tanur of their days, even if it is not swept or covered [and even if the food has not been cooked].”

[3] See Admur 253:21; Igros Moshe 4:74; Shabbos Kehalacha ibid; Admur 253:21 “[Furthermore] one is permitted to return it even on top of a Kirah that is swept or covered if it is not Yad Soledes in that area, even if one had already removed the food from there before Shabbos and had placed it on the ground and he did not have in mind to return it at all. This applies even if its food was placed into another pot, and even if it is an old food which has completely cooled off, and even if it has liquid.  The reason for this leniency when the coals in the oven have been removed is because when [on the oven] there is not so much heat to the point of Yad Soledes, it does not appear like one is initially cooking there [on Shabbos]. However [it is forbidden to place food] on top of an oven which is lit and has fiery coals inside of it [meaning they have not been removed or covered], even if it is not Yad Soledes on top of the oven and even if the food is completely hot. Although one may place a brick or something else under the pot and then place it on top of the oven [as will be explained in chapter 3 Halacha 3].” Vetzaruch Iyun why this is not subject to the same dispute mentioned above in 253:24 [brought in Halacha 4 above] regarding placing a food on top of a heater, in which the stringent opinion ruled one is not to do so unless there is an additional separation placed, even if the area is not Yad Soledes. However seemingly the difference is that by the case of a heater, since the inner fire was not covered therefore the stringent opinion suspects one may come to stoke the coals if no reminding separation is placed. However here that we are discussing an oven with covered coals, they no longer suspect one will come to stoke it, as is always the case when the coals are covered.

[3] Kitzur Diney Shehiyah and Hatmanah 259

The law by a Tanur and Kupach oven: A Tanur or Kupach with good fuel may never have food returned on it even if the coals have been swept and the area is not Yad Soledes as we suspect one may come to stoke the coals.

[4] 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.

[5] Admur 253:5 “(Regarding returning the pot to inside the Kirah, or Tanur even of our days, there is no difference if it is Yad Soledes or not in that area  and is thus forbidden on Shabbos, even if the food is fully cooked, [and] even if the oven [had its coals] swept or covered, [and] even if it is not Yad Soledes [inside the oven].)”; 253:19 “Every person should be stringent on himself to not return the pot to the inside of a Kirah or Tanur even if one removed it from there after Shabbos had begun, and even if he had intention to return it, and it is still in his hands.”; 253:21; M”A  319

Lenient custom in Admur: Admur 253:19 “There are some people who are accustomed to be lenient to return a pot on Shabbos day inside the Tanur which that food was baked in before Shabbos, even if [the coals of] the Tanur were not swept, [as long as] the Tanur was not further heated up after the baking [and is no longer Yad Soledes]. However, nevertheless if the inside of the oven is still Yad Soledes it is forbidden to place the pot there. The reason that they are lenient to allow this is because the coals which remain from the fire which was heated up for the baking are already faded and dimmed and they are thus considered as if they were swept. There are those which are furthermore lenient that [even if one had reheated the oven after the initial baking in order to] insulate the hot Shabbos day food nevertheless [they say that] after the Shabbos day meal certainly all the coals have dimmed and [they thus say that] it is permitted to return the food into that oven [after the Shabbos morning meal], in according to the custom [mentioned previously], (as long as the oven is not Yad Soledes). Despite the above custom, every person should be stringent on himself to not return the pot to the inside of a Kirah or Tanur even if one removed it from there after Shabbos had begun, and even if he had intention to return it, and it is still in his hands.”

Other opinions: Some Poskim rule it is permitted to return into the oven if it is not Yad Soledes. [M”B 253:93 in name of Gr”a; Shaar Hatziyon 253:93]

[6] Meaning that according to the lenient opinion there it would certainly be permitted to even initially place uncooked food into an oven which is not Yad Soledes.

[7] As writes Admur in 253:21 regarding a Kirah oven; However Tzaruch Iyun as the custom was only to allow returning  food into the oven and not to initially place.