Can a person use a thermos that was filled with a hot beverage before SHABBOS on Shabbos lchatchila? Is there a difference between SFARDIM and ASHKANAZIM in this regard? Should a person be strict?
Pouring water into a thermos before Shabbos is not an issue at all, according to all poskim. The reason is because even if a thermos would be considered hatmana, it is permitted to do hatmana in a container that doesn’t add heat before Shabbos. Regarding pouring the hot water into the thermos on Shabbos itself, according to the overwhelming majority of poskim, Askenazim and Sefardim, it is permitted l’chatchila. If you want to do it even according to the few poskim who rule stringently regarding this matter, so pour the water into a different bowl and then into the thermos, however as stated according to most poskim it is permitted.
It should be noted that if there is uncooked water on the bottom or even an accumulation of cold but cooked water on the bottom of the thermos, that it should be removed or dried before pouring water directly from the urn on to it.
Chelkas Yackov O:CH 84, Chazon Ish O:CH end of siman 37, Igros Moshe O:CH 1-95, Be’er Moshe 1-12, Minchas Shlomo 2-8, Oz Nidberu 1-48, B’tzel Hachochma 1-74, Yabia Omer 1 O:CH 14, Even Yisroel 9-8, Ohr Ltzion 2-17 (11), Tzitz Eliezer 11-29, Ashrei Hoish O:CH 2-4( 2), Shmiras Shabbos Khilchoso (new edition) 1-83, Orchos Shabbos 2-107, Piskei Teshuvos 257-9.