daf bit: Shabbat 36

The mishna teaches: if a double stove (meaning one that can hold two pots) is heated with stubble or rakings, a pot of already-cooked food can be placed there before Shabbat for use on Shabbat. But if the stove is heated with peat or wood, one has to sweep it or cover it with ashes before placing the pot. Beit Shammai says this is ok for a pot of water but not for food; Beit Hillel says even for food. Beit Shammai says one may remove it but not put it back (on Shabbat); Beit Hillel says one may remove and replace. (36b)

The reason given in the notes for the restriction on peat or wood is that these add heat (rather than just keeping the dish warm). Beit Shammai restricts to water because of a concern that one might rake the coals on Shabbat to more-thoroughly heat food. (I think that's what it's saying, anyway.)

This is all mishna; several pages of g'mara follow to expand on this.