May one immerse in a hot Mikveh on Shabbos?
The Sages forbade bathing one’s entire body, or majority of it, with hot water whether [the water] is within a vessel [or] whether it is in the ground, [and] even if it was heated from before Shabbos. Thus, the use of a hot Mikveh on Shabbos touches upon the Halachic question as to whether doing so transgresses the Rabbinical bathing prohibition. [This is aside for the Halachic discussion of the Kashrus of hot water for a Mikveh, even during the week, and the discussion of who is allowed to immerse on Shabbos, which are beyond the scope of this article.]
Warm: It is permitted to immerse in a warm Mikveh heated from before Shabbos, just as is the law regarding a cold Mikveh. Regarding the definition of warm versus hot: The Poskim define warm as water that is no longer referred to as hot by people, or water that is below 98.6 Fahrenheit, and thus it must be below this temperature to be allowed without question, otherwise it has the status of hot water.
Hot: Some Poskim prohibit immersing in a hot water Mikveh on Shabbos, based on the prohibition against bathing the majority of one’s body in hot water on Shabbos. This applies even to women who have a Mitzvah to immerse, and certainly applies to men. Other Poskim, however, permit immersing in even hot water for the sake of a Mitzvah [such as women immersing for Nida, or even men who immerse for Taharah or Baal Keri purposes]. Practically, the widespread custom is like this approach. However, some are initially stringent like the former opinion and immerse in only warm or cold water. Nevertheless, one may not remain in the water for pure pleasure purposes and is rather to immerse and immediately leave. Those who stay in the water more than necessary transgress the bathing prohibition according to all, and must be protested and told to leave. However, one may immerse in the water as many times as he is used to doing during the week.
Custom of the Sefaradim: The custom of Sefardi women is to immerse in hot water during Bein Hashmashos of Friday night. After Bein Hashmashos, they may only immerse in warm water. [Accordingly, Sefaradi men are certainly to avoid immersing in hot water on Shabbos.]
It is permitted to immerse in a hot Mikveh on Shabbos for the sake of a Mitzvah, such as for woman to immerse for Nida, or men to immerse for Baal Keri or even Tosefes Taharah. Nonetheless, it is forbidden to remain in the Mikveh to enjoy the water, and hence one is to enter, immerse and immediately leave. Those who remain in the water are to be protested. [It is best not to remain in the Mikveh after immersing, even if it is not defined as hot.] Some are particular not to immerse in any Mikveh defined as hot, and rather immerse only in cold water.
 Admur 326:1; M”A 326:2 in name of Kneses Hagedola; M”B 321:2
 Admur 326:1; Michaber 326:1; Shabbos 39b
The reason: The reason for this prohibition is because [back then] the people in charge of the bathhouses would heat up [the water] on Shabbos and claim that they heated it before Shabbos. Now, although the Jewish people are not [to be] suspected to advertently desecrate Shabbos, nevertheless these [people in charge of the bathhouse] would place wood [in the fire that is] under the cold water before Shabbos, close to dark, and the woods would burn throughout the entire Shabbos thus heating up the water that is above it. Now, [placing the wood so close to Shabbos] is forbidden because of a decree that one may come to forget and stoke the coals after Shabbos has begun. [Admur ibid [however see Admur 616:1 in parentheses]; Ran 40a, Rashba 40a in explanation of Gemara Shabbos 40a; Yerushalmi 3:3; See Tehila Ledavid 326:1]
Other opinions: Some Poskim rule the reason behind this prohibition is because one may come to heat up the water on Shabbos and transgress a Biblical prohibition of Sekila. [Implication of Admur 616:1 in parentheses; M”A 326:1; Olas Shabbos 326:1; Tosefes Shabbos 326:1; M”B 326:1; Kaf Hachaim 326:1] As the Balanim [caretakers of the bathouse] in previous times would heat up the water on Shabbos claiming it was heated before Shabbos. [Gemara ibid] They would place cold water into a Keli Rishon that is Yad Soledes which is Biblically forbidden on Shabbos. [P”M 326 A”A 4; Menorah hatehorah 326:1; See Shabbos Kehalacha 18 footnote 56]
Why does this not constitute a Gzeira Legzeira: It is learned from a verse that the Sages may not make a decree upon a decree. [Rashi Beitza 2b; See Admur 511:1] The explanation of Admur and Ran in the above decree form a decree upon a decree, and thus the question is asked as to how this was permitted to be done by the Sages. The following answers can be used to explain this matter: 1) It is permitted to make a decree upon a decree if the original decree will not be fulfilled without the second decree. [Tosafus Brachos 53a] 2) It is permitted to make a decree upon a decree when the original decree is due to a suspicion that involves the item to which the action is being done, such as insulating with items that add heat is forbidden because one may come to insulate with ember which can lead one to stoke the ember. Since the decree against ember involves stoking itself, therefore making a further decree against all material that adds heat is not defined as a decree upon a decree. [Tosafus Rid Shabbos 31a] 3) Any matter which is commonly transgressed the Sages may make a decree upon a decree. [Chacham Tzvi 75; Gra 252]
 Regarding the Kashrus of a hot Mikveh from the aspect of the laws of Mikvaos: Some Poskim forbid immersing in a hot Mikveh even during the week. [Yeish Mi Sheomer in Michaber Yoreh Deah 201:75; Mordechai Shavuos 2 in name of Rabbeinu Tam] This is due to that people coming to think that a bathtub is also a Kosher Mikveh. [Taz 201:88; Mordechai ibid, brought in Beir Hagoleh ibid] Other Poskim, however, rule that it is permitted to immerse in a hot Mikveh. [Opinion in Rama 201:75; Hagahos Mordechai in name of Ravayah and Riva] The Rama ibid concludes that one is to be stringent not to do so, however, those places which are lenient to do so are not to be protested. [Rama ibid; Binyamin Zeev] The Alter Rebbe directed and instituted to heat the Mikvaos [See Tikunei Mikvaos where Admur writes that hot water is placed into the Mikveh; Beis Rebbe p. 32 that this was one of the Takanos of Admur; Shut Tzemach Tzedek Y.D. 172; 176; 334; See Tikunei Mikvaos Levin p. 16] Practically, so is the custom today of all Jewry [including Sefaradim] to permit immersing in hot Mikvaos. [See Rav Poalim Y.D. 4:15 in length regarding custom of Sefaradim; Rav Ovadia Yosef in Taharas Hbayis 2:15]
 See Admur 326:7; Michaber 326:8; Beitza 17b
 Admur in supplement from Mahadurah Basra chapter 259 Page 884 in the new Shulchan Aruch; Chacham Tzevi 11 which forbids for women to immerse in water on Shabbos that is called hot, rather it must be cold or slightly warm; Nodah Beyehudah Tenyana Orach Chayim 24 that women may bathe in warm Mikvaos but not hot Mikvaos; Aruch Hashulchan 326:3; P”M 511 M”Z 5; Ashel Avraham Butchach 326; Tehila Ledavid 326:3; Minchas Shabbos, brought in Ketzos Hashulchan 133 footnote 1; Shevisas Hashabbos Mivaehseil 125; M”B 326:7; Igros Moshe 1:126; SS”K chapter 14 footnote 3; See Piskeiy Teshuvos 326:1; Shabbos Kehalacha 18:19 Tosefes Biur 1 for all the opinions on this matter
Other Poskim: Some Poskim rule that warm water is forbidden to bathe in just like hot water [Beis Meir Y.D. 197:3; Rambam brought in Biur Halacha; However the M”B himself 326:7 writes that bathing in warm water is allowed [for Mikveh]. Some however write that in truth the M”B holds that only water in which some coldness is felt is allowed, and it is by this warmth that he allows to bathe, however water which its heat is felt is forbidden as writes the Rambam, as he brings in Biur Halacha. [Az Nidbaru, brought in Piskeiy Teshuvos 326:2].
 See Aruch Hashulchan 326:3; Chacham Tzevi 11 who forbids for women to immerse in water on Shabbos that is called hot, rather it must be cold or slightly warm; Zera Emes 71;
 Tehila Ledavid 326:3; Ketzos Hashulchan 133 footnote 1 in name of Minchas Shabbos; Minchas Yitzchak 4:44; Or Letziyon 2:35-3; See SS”K chapter 14 footnote 3
 See Pischeiy Teshuvah Yorah Deah 197:1 for a compilation of different opinions.; Shabbos Kehalacha 18:47; Piskeiy Teshuvos 326:5
 Chacham Tzevi 11; Nodah Beyehudah Tinyana Orach Chayim 24 [so writes Pischeiy Teshuvah as well as Divrei Chayim, however Tzitz Eliezer argues on their opinion in Nodah Beyehudah.]; Chayeh Adam 70:1; Chochmas Adam 122:22; Aruch Hashulchan 326:10; Rav Poalim O.C. 4:15; M”B 326:7 in name of Chayeh Adam; Igros Moshe 4:74; Or Letziyon 2:35-3; The Sheivet Halevy [5:44] rules that it is best to immerse in a warm Mikveh if he is able to, despite the fact that the custom is to be lenient.; See Poskim in Piskeiy Teshuvos 326:5 footnote 58
 Karban Nesanel [Perek Bameh Madlikin 22:11] says that this is not included in the decree against bathing; Bircheiy Yosef Yoreh Deah 197:2; Divrei Yosef 64, although he concludes with a Tzaruch Iyun [so says Pischeiy Teshuvah, however Tzitz Eliezer differs that he does allow it and so learns others as well]; Sidrei Taharah Y.D. 197:16; Chesed Lealafim 326:7; Divrei Chayim 2:26, Avnei Nezer 1:526; Darkei Teshuvah 197:7; Levushei Mordechai 1:1; Maharshag 2:127; M”B 326:7 brings the Karban Nesanel and in Shaar Hatziyon 326:5 writes one may be lenient in a time of need; Kaf Hachaim 326:33; Minchas Yitzchak 5:32; Tzitz Eliezer 6:22 brings many opinions which permit it; Divrei Yatziv 160-161; Orchos Rabbeinu 1:152 writes that Steipler would immerse in hot Mikveh if nothing else available; Sheivet Halevy 5:44 rules that it is best to immerse in a warm Mikveh if he is able to, despite the fact that the custom is to be lenient; See many Poskim in Piskeiy Teshuvos 326:5 footnote 54
 The Karban Nisanel ibid does not explicitly state why a Mikveh is not included in the decree of bathing, however the other Poskim who discuss his ruling record several reasons: 1) As many people are there and one will not come to transgress; 2) As it is done for the sake of a Mitzvah. 3) A mere dip is not considered bathing. Practically, the second reason is the main reason recorded behind the basis of this custom. [See Noda Beyehuda ibid; Avnei Nezer ibid; Divrei Yatziv ibid; Igros Moshe 1:126; Piskeiy Teshuvos 326 footnote 51]
 While many of the above Poskim discuss specifically a woman’s immersion it is evident that this would apply likewise to a man. Furthermore, some of the above Poskim explicitly write that this extends also to the immersion of a man. [See Chesed Lealafim ibid; Aveni Nezer ibid who permits but says Lechatchila is best for man to avoid if just for Tosefes Taharah; Divrei Yatziv ibid; Piskeiy Teshuvos 326 footnote 54 and 58]
 Divrei Chaim ibid
 See Piskeiy Teshuvos 326:5
 Avnei Nezer 1:526 “To sit in the hot Mikveh after immersing is certainly forbidden, however to enter, immerse and immediately leave has room for Halachic leniency” [However, based on reason 1 and 3 recorded above, even this would be allowed. However, practically the main reason is reason 2, and it is hence only permitted for the sake of a Mitzvah.]
Does this apply even if the Mikveh is warm? Seemingly, even if the Mikveh is not hot, but warm or cold, one is to leave right away, as it is nevertheless forbidden to bathe in even cold water due to custom. [See Lehoros Nasan 4:30; Piskeiy Teshuvos 326:5] However, perhaps we say that once one has entered with permission, there is no longer a custom to prohibit remaining a little longer. Vetzaruch Iyun
 See Piskeiy Teshuvos 326 footnote 56
 See Zera Emes 1:71; Rav Poalim 4:21; Or Letziyon 2:35-3; Yabia Omer 10:55; Darkei Halacha p. 169; Piskeiy Teshuvos 326 footnote 54