Siman 258

The Rambam wrote at the end of chapter 4: "One may place a water heater (of copper) on top of a water heater on Shabbat, and a pot (of earthenware) on top of a pot, and a pot on top of a water heater and a water heater on top of a pot, and seal their mouths with dough. Not in order that they should heat up, but in order that they should remain at their heat. For they only forbade to insulate on Shabbat, but to place a hot vessel on top of a hot vessel so that they remain in their heat—it is permitted. But one may not place a vessel containing a cold substance on top of a hot vessel on Shabbat, for behold, he generates heat in it on Shabbat. And if he placed it from the eve of Shabbat—it is permitted, and it is not like one who insulates with a substance that adds heat." Thus far his words. (And such is the version in the Gemara at the end of the chapter 'Bameh Tomnin,' and not like the version before us that a water heater on a pot is forbidden, and also Rashi there wrote so like the Rambam according to the Tosefta, see there.)

The explanation of the matters: For one who places a hot vessel on top of a hot vessel—there is no issue of cooking, since both are hot and equal in their heat, but if they are not equal, it will be explained in siman 318; and there is also no issue of hatmanah, since the upper one is uncovered—this is not hatmanah. And it is also not similar to shehiyah on top of a kirah, for there, there are coals and the heat is great, which is not the case with a meicham on top of a meicham. And only a cold item is forbidden, because it generates heat, but from Erev Shabbat it is permitted, because this is not hatmanah, as I have written. And it is obvious that what was written, "and plastered their openings with dough," refers to when the dough was kneaded from Erev Shabbat, and the plastering is permitted because it is not for permanence but only for a short time. And it is obvious that when the pot stands on the fire—it is forbidden to place another pot or meicham upon it, for this is actual shehiyah. (Magen Avraham) And there is another reason why hatmanah does not apply to this, because the heat continually cools down. (Beit Yosef) And even though if so, ashes and coals also continually cool down, nevertheless their heat is very strong, and it is not comparable to the heat of hot water. But in truth, this is only necessary according to the opinion explained that placing the bottom of the pot on the coals constitutes hatmanah, but it has already been explained that we hold that as long as the entire pot is not buried within it—it is not hatmanah; and see in siman 318, section 36.

There is one who hesitates regarding people who, during the winter days, take a vessel containing a cold drink and place the vessel inside a vessel full of hot water to temper its coldness, for this is complete hatmanah. (Taz) And we have not heard of anyone who does so; we have only heard this: that there are women who take the vessel containing the kugel and place the entire vessel inside the large pot of cholent, and this is complete hatmanah in a substance that increases heat, since the pot is inside the oven which is full of coals, and it is forbidden even from while it is still day. Unless we say according to the reasoning of the Rosh that we wrote in the previous siman, section 14, based on the reason that the pot was previously in the oven and based on the reason of being attached, see there; and certainly it is better not to do so. (Or because the oven is plastered and even with hatmanah it is permitted just as with shehiyah.)

Also, one must be careful among us regarding the drinking of hot water on Shabbat morning, for they place a clay vessel or a copper vessel with water in the oven, which they call "leyak," and they also place the "tshaynik" with the tea in the oven, and they take them from the oven in the morning and drink. They must be careful not to cover neither the "leyak" nor the "tshaynik" with a cloth or another object, for this is a complete act of hatmanah ab initio on Shabbat in a substance that does not add heat, and it is forbidden as has been explained.