I listened to a shiur recently and it came out that it would be assur to eat food while it is hot if it was heated in an oven on shabbos day and you would have to wait for it to cool down so that you aren’t benifitting from their melacha. He said that although this type of chazara is only a d’rabanan, because it’s bishul it would still be a problem. Obviously he said some other stuff, but I would be interested to hear why it’s not a problem of ma’aseh shabbos.
Thanks very much
The laws of benefiting from forbidden Shabbat activities (ma'aser shabbat) are outlined in the Shulchan Aruch 318,1. There you will find that there is a difference between Rabbinic and Torah desecrations, with the law relating to benefiting from Rabbinic sins much more lenient. It would follow then that one would think that if food was reheated in a Rabbinically forbidden way, unintentionally, it should be permitted. However, as you heard in the shiur you listened to, the laws relating to benefiting from forbidden reheating of food are stricter (see the Shulchan Aruch and Rema in 253,1).
My ruling that one may eat the hot food that was reheated on Shabbat even in an oven (unless one knows that the oven was lit on Shabbat), was based primarily on the Biur Halacha who states that the strictures of benefiting from reheating only apply when the reheating is forbidden according to all opinions (Biur Halacha 253,1, "V'Im Hechziro Yisrael"). Because there are opinions which are very lenient about reheating in an oven (even foods with gravy – according to the opinions who define solid food according to the majority of the food), even though the Rema and Shulchan Aruch have already ruled not to rely on these opinions, after the fact the food is permitted. [There are also opinions that allow the fully cooked food to be placed into the oven on Shabbat when the oven is off, even though it will be turned on later by a Shabbat timer - which may have been the case in the question I was asked, and this possibility was a consideration in my ruling]. Also, see the Mishna Brurah ibid, 35, who rules that even when the food is forbidden because of forbidden chazara (returning to the fire), it is only forbidden for the person who did the act and his family, but not for others. (The Rabbi in the shiur you heard was probably talking about totally forbidden reheating according to all opinions, and for the re-heater themselves, or their families - which was not the case in the question I was asked, where the guest wanted to know what to do).
I hope all this helps you in your learning of the laws of Shabbat - of course if you need a ruling about how to heat up food on Shabbat, or what to do if the halacha was not observed, you should feel free to ask us again for a ruling.