The Halachot of Hatmana touch on may practical issues. One example discussed by the poskim is warming a baby bottle by immersing it in a bowl of hot water. Since the hot water is a Keli Sheni, there is no issue of Bishul (Cooking). The question is does the fact that the bottle is "insulated" and surrounded by the hot water present a problem of Hatmana.
Clearly, the hot water is not considered Mosif Hevel, heat generating insulating; the water temperature is constantly dropping. Nonetheless, Hatmana on Shabbat is prohibited even with non- heat generating substances.
Nevertheless, Hacham Ovadia permits fully immersing the bottle in the Keli Sheni. This leniency is based on the ruling of Maran (Orah Chayim 318:13, 257:6) that insulating cold items, such as the bottle, is less problematic than insulating hot items. This is also the opinion of the Eliya Raba (Commentary on the Shulhan Aruch by R. Eliyahu Spira, Prague, 1660–1712) and the Zera Emet Halachic work by R. Yishmael HaKohen 18th Century Italy) who quotes from Rishonim such as the Riaz.
Hacham Ben Sion Abba Shaul (Israel, 1923-1998) disagrees and takes into account the stricter opinion of the Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933 in siman 258), the Taz (Rabbi David Segal, Poland, 1586-1667), and the Rashba. Hacham Ben Sion concludes that it is forbidden to fully submerge the bottle. It is permitted to put the bottle in hot water, only if part of the bottle remains above the surface. That way, it is only considered a Hatmana B’Miktzat, a partial insulation.
While one can rely on the leniency of Hacham Ovadia, nevertheless, it is preferable to adopt the ruling of Hacham Ben Sion, as it is not difficult to insure that the upper edge of the bottle remains above the surface.
It is permissible to immerse a cold baby bottle in hot water of a Keli Sheni. Preferably, part of the bottle should remain out of the water.