Q. Our water kettle stopped working on Shabbos and we went to the neighbors to get some hot water. Can you cover the utensil with a towel to keep the water hot? Can you use a thermos flask if you leave it uncovered?
A. Shulchan Aruch (O.H. 257: 5) rules that the Rabbinical prohibition of hatmono or insulating food on Shabbos applies only to a keli rishon or the vessel the food was cooked in. If it was transferred to a secondary vessel or keli sheni, it may be insulated and kept warm with materials that do not increase the heat of the container.
Many Poskim are lenient in the use of a thermos flask, even if lid covered, as it is a secondary utensil. (Chelkas Yaakov O.H. 84, Igrois Moshe 1:95, Betzel Hachochmo 2: 74: Be’er Moshe 1: 12, Minchas Shlomo 2: 8, et al.).
Horav Shlomo Miller’s Shlit’a opinion is similar
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a