Using a Thermos on Shabbos

Can I pour hot coffee into a thermos on Shabbos and put the plastic lid on it?

Can I pour hot coffee into a thermos before Shabbos and open and close it later on Shabbos?

Answer:

There is no problem with pouring hot liquid into a thermos on Shabbos, or opening and closing its lid on Shabbos. The reason for this is that the prohibition of hatmanah applies only when the vessel containing the food or liquid is wrapped by another layer. In the case of a thermos, the entire thermos (even though it possesses two layers) is seen as one vessel, and there is therefore no prohibition of hatmanah (Chazon Ish Orach Chaim 37). Before pouring in the hot liquid, one must ensure that the cup is dry.

If the liquid has been poured in before Shabbos, there is no prohibition of opening and closing the lid.

The problem with closing a lid is that it causes the food to cook. In our case this does not apply for two basic reasons: 1. The liquid in the thermos is fully cooked, and closing the lid cannot cook it beyond its present state (Mishnah Berurah 254:23; see Shemiras Shabbos Kehilchasa chap. 1, note 94; 2 — yet as Teshuvos Ve’Hanhagos 1:207 writes, the custom is to be lenient); and 2. According to most authorities a thermos is considrered a keli sheini (Shemiras Shabbos Kehinchaso 1:70; but see Menuchas Ahava, vol. 2, 10:24), which does not have the ability to cook.