Answer: In the previous Halacha, we have explained that according to Maran Rabbeinu Ovadia Yosef zt”l, one may use hot water heated by a solar water heater (common in Israel) to wash one’s hands, dishes, and the like. However, washing one’s entire body (or even a majority of one’s body) with hot water is forbidden on Shabbat. Even if the water was heated before the onset of Shabbat and even if was heated by a solar water heater, there is nevertheless a rabbinic enactment prohibiting the washing of the majority of one’s body with hot water on Shabbat based on the Baraita (Shabbat 39b). The Gemara there explains that the reason for this enactment was because originally, it was customary to bathe in hot water on Shabbat and as a result of this, the Sages saw that the bathhouse attendants would transgress several prohibitions and desecrate the Shabbat in order to heat the water at which point they then decreed that it was forbidden to bathe in hot water on Shabbat. As a result, all bathing in hot water became forbidden, even if the water has been heated in a permissible manner, such as before the onset of Shabbat or by a solar water heater. Even if one does not wash one’s entire body at once and merely does so part by part, this is nevertheless forbidden. Nonetheless, washing only a small portion of one’s body is permissible when using water heated before the onset of Shabbat or water heated by a solar water heater, as we have explained in the previous Halacha. However, it is completely forbidden to use water heated by an electric or gas (with a pilot, common in the United States) boiler on Shabbat because of the prohibition of cooking on Shabbat.
Thus, if a woman must immerse in the Mikveh on Shabbat night and the water in the Mikveh is hot and she can neither immerse in such water on Shabbat because of the rabbinic enactment prohibiting bathing in hot water nor before the onset of Shabbat (as one must wait until night following the seventh clean day to immerse), according to Maran Rabbeinu Ovadia Yosef zt”l, such a woman must immerse during twilight of Friday evening, i.e. any time between sunset and approximately thirteen and a half seasonal minutes past sunset, for there is still considered a doubt during this time whether or not Shabbat has begun. Since the prohibition to bathe in hot water on Shabbat is not a Torah prohibition and is merely rabbinic, there is room to allow doing so during this time span during which a doubt still exists as to whether or not Shabbat has entered. This is indeed the most halachically preferable solution to immersing in a Mikveh on Shabbat night. When Maran zt”l first issued this ruling many years ago, it was still customary for woman to only immerse in the Mikveh on Friday evening after the period of twilight and the Mikveh attendants would not allow women to immerse during twilight. Maran zt”l went from place to place and from city to city where he proceeded to summon the various Mikveh attendants and instructed them to allow women (primarily Sephardic women) to immerse in the Mikveh at this time so that they would not be forced to immerse in hot water on Shabbat. Thank G-d, Maran zt”l’s ruling has spread all over the world regarding this issue and the vast majority of Mikveh attendants, especially in communities where there is a large contingency of Sephardic Jews, allow women to immerse in the Mikveh on Friday evening immediately following sunset and as a result, women who need to do so avoid any unnecessary halachic problems. Nevertheless, if for whatever reason a woman was unable to immerse in a hot Mikveh during the time of twilight on Friday evening, Maran zt”l rules that this is not reason enough to push off her immersion to a different night and she may act leniently and immerse in the Mikveh even after the time of twilight has passed since there is no other alternative.
Important Note: Based on the above, although there is room for leniency for Ashkenazi men to immerse in a hot Mikveh on Shabbat, according to the Sephardic and Middle Eastern tradition, there is absolutely no room to allow men to immerse in a hot Mikveh on Shabbat due to the rabbinic enactment forbidding bathing in hot water. If a Sephardic man wishes to immerse in a Mikveh on Shabbat, he may do so only in a Mikveh where the water is cold (as we shall discuss in further in the following Halacha).
In the following Halachot, we shall discuss whether or not certain individuals may act leniently and wash their bodies with hot water on Shabbat.
