Question:
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?
Answer:
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.
Best wishes
Sources:
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.
May I suggest you update this post — Your heter of pouring water into a thermos on Shabbos can turn into a michshal for people who don’t dry the thermos completely before pouring in boiling water. With a narrow-mouthed thermos, it’s impossible to be confident that the thermost is absolutely dry inside.
To avoid bishul achar bishul, there are conditions that have to be met. Drying the thermos is essential, but virtually impossible.
I saw this question originally in print, from the weekly parsha sheet. I searched in google to find the post on the website so I could ask this question. But for those who read this question printed, they will not see your update, and they will continue to add boiling water to thermos bottles that may — or may not be dry.
Interesting point. The thermos only has to be dried completely if it was washed out with cold water that was not previously heated (washed out with cold water). If it wasn’t washed out since the last tea, then as long as there isn’t an accumulation of water there is isn’t an issue, (see Igros Moshe O:CH 4 74 Bishul 19).
Additionally, you chashas only applies if the stream of hot water directly hits the uncooked cold water at the bottom of the thermos, and it is quite likely that the water first hit the sides of the thermos, (as it is narrow). Another point, where do you get this assumption that a thermos cannot get totally dried out. If it I turned upside down after washing it and left like that for a little while, all of its water will drip out or evaporate.
Nevertheless, I will add the point that the one should be careful not to add hot water to a thermos that has uncooked water, or an accumulation of cooked water on the bottom of it.
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