A “Keli Rishon” – a utensil in which food or liquid has been cooked – has the capacity to effectuate “cooking” in the Halachic sense of the term. This means that placing raw food inside a “Keli Rishon” on Shabbat violates the Torah prohibition of cooking on Shabbat. This applies even after the utensil has been taken off the fire, so long as it is still hot. However, if one transfers the contents of a “Keli Rishon” to another utensil, that second utensil – the “Keli Sheni” – does not have the capacity to cook. The walls of that utensil cool the hot liquid such that it is no longer capable of “cooking” as defined by Halacha. Thus, if one pours boiling water from its original pot into another utensil, one may cook raw food in the water in the second utensil.
Rabbi Moshe Halevi (Israel, 1961-2001), in his work Menuhat Ahaba (vol. 2, p. 340), rules that this Halacha of “Keli Sheni” does not apply to a thermos. A thermos is specially insulated to retain the heat of its contents. As such, hot liquid poured from a “Keli Rishon” to a thermos cannot be assumed to have cooled, as it does in a regular “Keli Sheni.” We must therefore treat the thermos as a “Keli Rishon,” and it would be forbidden to place raw food or a teabag inside the thermos.
However, Rabbi Moshe Halevi adds, it would be permissible to pour hot water from a thermos onto a teabag to make tea. He reaches this conclusion on the basis of a combination of two factors. First, some Rishonim (Medieval Halachic authorities) maintain that liquid poured from a “Keli Rishon” does not have the capacity to cook; in their view, hot liquid can cook only while it is inside the original pot. Secondly, the water poured from a thermos into a teacup has come in contact with air twice – after leaving the original pot, and when leaving the thermos. This allows us to treat water poured from a thermos more leniently than water inside a thermos. Hence, although one may not place a teabag inside a thermos, one may pour hot water from a thermos onto a teabag in a teacup to make tea. This is also the ruling of Hacham Ovadia Yosef, as published in the pamphlet Kol Sinai.
This Halacha applies as well to an insulated tiger pot which people often fill with hot water from an urn and then bring to the table. It is permissible to pour from the tiger pot directly onto a teabag to prepare tea.
Summary: One may not place raw food inside a thermos or tiger pot of hot water, but one may pour hot water from a thermos or tiger pot onto raw food, such as a teabag.