A Thermos on Shabbos
ANSWER: The Chazon Ish (OC 37:32) offers two reasons why this is permitted. First, he writes that a thermos is considered a kli sheini (second vessel that was not heated on the fire). Shulchan Aruch (257:5) writes that it is permissible to do hatmana on a kli sheini. Since an action was done to cool down the food, we are not concerned that one will place it back on the fire. Second, he writes that putting something into a thermos is not hatmana. Hatmana refers to wrapping a pot with another layer, such as a towel. However, a thermos is a utensil which has heat resistant walls – just as one may pour food on Shabbos into a thick-walled pot, so too one may pour into a thermos.
Shevet Halevi (1:93) questions these reasons. He argues that a thermos cannot be compared to a kli sheini, since the purpose of the thermos is to keep the food hot. This is not the same as pouring liquid into a kli sheini which actively cools down the food. Regarding the second point, he questions if walls of a thermos which are made specifically to insulate are the same as a thick-walled pot.
Igros Moshe (OC 1:95) offers another rationale to justify pouring hot liquids into a thermos. Hatmana is only prohibited when the entire vessel is wrapped. Therefore, at the time when one pours hot water in the thermos there is no violation of hatmana since the top is uncovered. Putting a cover on the thermos also cannot be treated as hatmana because the purpose of doing so, in addition to maintaining the heat, is to protect the liquid from gathering dust or from spilling out.
Although it is forbidden to eat before davening, Shulchan Aruch (OC 89:3) writes that one may drink water. Poskim extend this leniency to drinking tea or coffee as well. Mishnah Berurah (89:22) writes that one may not add milk and only permits a very minimal amount of sugar, but the Aruch Hashulchan (89:23) writes that the prevailing custom is to be lenient regarding adding sugar and milk to coffee as well, since people find it difficult to drink tea or coffee otherwise. However, one should only add the minimum amount that they need.
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The Gerald & Karin Feldhamer OU Kosher Halacha Yomis is dedicated to the memory of Rav Yisroel Belsky, zt"l, who served as halachic consultant for OU Kosher for more than 28 years; many of the responses in Halacha Yomis are based on the rulings of Rabbi Belsky. Subscribe to the Halacha Yomis daily email here.