On Erev Shabbat, it is prohibited to use heat generating materials to insulate a pot of food. The Rama (Rabbi Moshe Isserles, Cracow, 1520-1572, Siman 257) cites a lenient opinion that if one is insulating food on Erev Shabbat L’Sorech Machar, to remain hot until tomorrow morning, he would be allowed to use even heat generating materials. The rules of Hatmana would not apply to such a case because he has taken his mind off the food until morning. Therefore, we are not concerned that he will come to do prohibited activity.
Maran does not agree with this position. He holds that the Halachot of Hatmana apply even if the food is meant for the next day.
Hacham Ovadia, in Hazon Ovadia, rules like Maran and does not rely on the Rama’s leniency. However, he is willing to use this leniency in conjunction with an additional leniency. While each leniency by itself would not be sufficient to rely on, together they form a solid a Halachic basis.
For example, there are opinions that it is permitted to insulate, if one does not fully cover all sides of the pot. We do not hold like those opinions. However, we can combine those opinions with the opinion of the Rama that L’Sorech Machar, insulating for the next day, is permitted. That is, it is permitted to insulate on Friday without covering the entire pot for the purpose of eating the food tomorrow.
SUMMARY: One is permitted to insulate a pot on Erev Shabbat with heat generating materials if the following two conditions are met:
1. The food in the pot is meant to be consumed the next day, on Shabbat morning.
2. The Insulation does not fully cover all sides of the pot.