[Halachically Speaking appears on YWN weekly, Tuesdays and Thursdays]
Written by Rabbi Moishe Dovid Lebovits of KOF-K Kosher Supervisio
The posuk in the Torah says that melacha is generally forbidden on Yom Tov, although melacha which is needed to prepare food to eat is permitted. Once these melachos (cooking, and baking) were permitted for the preparation of food they are also permitted for other melachos. Accordingly, cooking and heating up water on Yom Tov is permitted.
However, melachos for other purposes are only permitted if they are shoveh lechol nefesh – something which is important to most people. Therefore, one may only heat up water on Yom Tov if it will be used for a purpose which is shoveh lechol nefesh.
Washing the Body
The Shulchan Aruch rules that one may not heat up water on Yom Tov in order to wash his entire body, since doing so is not shoveh lechol nefesh. Heating up water to wash one’s hands, face, and feet is permitted, since doing so is considered shoveh lechol nefesh. Since washing the whole body is not shoveh lechol nefesh, one may not take a shower on Yom Tov. This prohibition applies even if the water was heated up before Yom Tov since one may come to heat up water on Yom Tov. 
Washing part of the body
One may use water which was heated up before Yom Tov to wash one limb at a time, even if he will end up washing all of his body. Additionally, some poskim permit one to use water which was heated up on Yom Tov to wash one limb at a time, as long as one does not end up washing most of the body, since washing most of the body will be considered as if one washed the whole body (which is not shoveh lechol nefesh). However, if the water was heated from before Yom Tov, we do not consider washing most of the body as washing the whole body.
Does Shoveh Lechol Nefesh Change?
The poskim discuss whether or not shoveh lechol nefesh can change with time. In earlier times when houses did not have indoor plumbing, people generally did not shower on a daily basis. Therefore, washing the whole body was not considered shoveh lechol nefesh. The question is since most people today wash their whole bodies (i.e. take a shower on a daily basis) is doing so considered to be shoveh lechol nefesh, or are the rules set down by chazal regarding shoveh lechol nefesh unchangeable? L’maseh, the opinion of the poskim is that despite all this the gezeirah ofchazal is still in effect and unchanged.
Face, Hands, and Feet
As mentioned above, one is permitted to heat up water even on Yom Tov to wash his face, hands, and feet. Some say one may only wash the face and not the entire head. When washing the hands, one may wash until the elbow, and when washing the feet, one may wash until the ankle.
One may not walk into a shower (even if he only intends to wash part of his body) since we are concerned that he will come to wash his entire body. Therefore, if the limbs of the body which he is not allowed to wash are covered with a garment, one may walk into a shower to wash the uncovered limbs. Alternatively, one may stand outside the shower and stick his face, hands, and feet into the shower to be washed.
There is an uncertainty if a person who is very dirty may shower his entire body on Yom Tov.
Modern Water Heaters (Boilers) on Yom Tov
Our modern water heaters work in the following manner. Water is heated up to a certain temperature at which it is maintained automatically. When one uses hot water, new cold water enters the boiler causing the hot water in the tank to cool that is detected by the thermostat, which then causes the flame to re-heat the water. Accordingly, anytime one removes hot water onYom Tov one causes new water to be heated up in its place. Therefore, all hot water in our modern day boilers is considered to have been cooked on Yom Tov, since one uses water on Yom Tovnew water is getting cooked up in its place. Therefore, one would not be able to use any water from a modern water heater to shower or wash most of the body on Yom Tov.
“I Need a Shower”
Many times people say they need to shower on Yom Tov because they “can’t do without one” or they are a “istinus” for whom showering is permitted. Such statements have no basis in halacha. The whole reason why it is forbidden to cook up hot water for a shower on Yom Tov is because it not shoveh lechol nefesh since only an istinus needs one everyday. How then can such reasoning be used to permit a shower? This argument will refute such statements even during a three-day Yom Tov. Others are lenient in this regard as well.
Giving a child a bath or shower is only permitted where the water has been heated on Yom Tov for a permitted purpose. One is then permitted to use such water to wash his child. However, placing the child into a bath or shower with water that is heated up on Yom Tov specifically for the child is forbidden. One should consult his local halachic authority whether water taken from a modern-day water heater is considered as having been heated specifically for the child.
In any case, washing a child in the permitted manner may only be done if the child receives a bath on a daily basis.
When washing a child in the permitted manner one is allowed to place a towel under the child’s body even if it will get wet.
One who is very hot is permitted to take a cold shower on Yom Tov. One should not take a lukewarm shower on Yom Tov since one has to turn on the hot water for this purpose, which is not permitted when one would be washing his whole body.
One is not allowed to use a bar of soap on Shabbos or Yom Tov because doing so involves the melacha of smoothing (me’machek). Most poskim maintain that one is permitted to use liquid soap on Shabbos or Yom Tov. However, Horav Moshe Feinstein zt”l was stringent. One is not allowed to use a sponge when washing his body on Yom Tov because of the problem of sechita. 
When hair is washed on Yom Tov (i.e. when one went to a mikvah), one must be careful not to perform sechita on his hair by drying it vigorously with a towel. Some say drying hair with a towel is permitted because it is being done k’lacher yad, and the sechita is only a d’rabbanan since the water is not being used but is going to waste. Some people are concerned even when drying the hair in a slow manner because it still may cause one to do sechita. It would seem that according to this opinion one may only let his hair dry by drip drying without using a towel at all. L’maseh, those who dry their hair with a towel have whom to rely on. According to the opinion that permits using a towel, one may also use a paper towel even if it will get ruined and ripped during the drying process.
According to some poskim, sechita does not apply to the hair on one’s hands, arms, feet or eyelids.
 Sefer Shemos 12:16.
 Refer to Rambam Hilchos Yom Tov 1:5, Shulchan Aruch 495:1, Rama, Pri Megadim Eishel Avraham 511:4, Shita Mekubetzes Kesubos 7a, Aruch Ha’shulchan 511:12, Encyclopedia Talmudis 1:27:footnote 12, Birchos Avraham Beitzah page 58.
 Mesechtas Kesubos 7a.
 Refer to Tosfas Mesechtas Beitzah 21b “lo,” Ran, Rambam Hilchos Yom Tov 1:16, Tur 511, Shulchan Aruch 511:2, Levush 2, Mishnah Berurah 9-10, Kaf Ha’chaim 12, Shulchan Shlomo 511:2:4, Shevet Ha’kehusi 1:159:4, see Mesechtas Beitzah 21a. Refer to Moadim V’zemanim 1:45 who explains the opinions in this inyun.
 Refer to Mesechtas Kesubos 7a, Tosfas “m’toch,” Pri Megadim introduction to Hilchos Yom Tov 1:1, Shulchan Aruch Harav 511:2 (kuntres achron), Tosefta Chazzon Yecheskel Beitzah 2:7:page 76. See Chinuch mitzvah 298.
 Pri Megadim Eishel Avraham 511:4. Refer to Bais Meir Y.D. 197:3.
 Refer to footnote 4.
 O.C. 511:2, see Mesechtas Beitzah 21b.
 Tosfas ibid, Rambam ibid, Prisha 2, Shulchan Aruch 511:2, Levush 2, Magen Avraham 4, Taz 2, Sharei Teshuva 511, Sdei Chemed Yom Tov 7:page 213, Mishnah Berurah 10. Refer to Betzel Hachuchma 2:31:pages 61-63.
 Rama 511:2, Levush 2, Kaf Ha’chaim 30, Aruch Ha’shulchan 4, Be’er Moshe 8:158. Refer to Shibuley Haleket 243:page 224, Shulchan Aruch 511:2. The Rambam ibid says washing ones whole body is forbidden because of a gezeirah. (Refer to Taz 4, Levush 1, Kaf Ha’chaim 13). The Mechaber ibid is lenient if the water was cooked up before Yom Tov.
 Mishnah Berurah 18, Kaf Ha’chaim 28, Shemiras Shabbos K’hilchoso 14:7, Avnei Yushfei 3:55:3.
 Rashba Mesechtas Shabbos 39b, Biur Halacha “avol,” Kaf Ha’chaim 14, Be’er Moshe 8:169. See Pri Megadim M.Z. 511:4. One may heat up hot water to make the cold water warmer since that is shoveh lechol nefesh (Shar Ha’tzyion 511:25).
 Refer to Biur Halacha 511 “yudov.”
 Refer to Shemiras Shabbos K’hilchoso 14:footnote 21, 19:footnote 3 quoting the opinion of Horav Shlomo Zalman Aurbach zt”l, Rivevos Ephraim 6:265, 8:248:1:page 276, Shulchan Shlomo 511:2:3. Some say even a shower once a week may be considered equal to all (Shulchan Shlomo 511:footnote 3). Parenthetically a five minute shower uses about 15-25 gallons of water.
 Horav Yisroel Belsky Shlita, Shemiras Shabbos K’hilchoso 14:footnote 21, Rivevos Ephraim ibid, Be’er Moshe 8:158-159, Tzitz Eliezer 6:20, 11:64, Opinion of Horav Elyashiv Shlita quoted in Avnei Yushfei 3:55.
 Horav Yisroel Belsky Shlita.
 Horav Yisroel Belsky Shlita. Refer to Shulchan Shlomo 511:2:3:2.
 Mishnah Berurah 9, Piskei Teshuvos 511:footnote 25, The Laws of Yom Tov page 201.
 Natei Gavriel Yom Tov 31:2.
 Refer to Shulchan Shlomo 511:2:3.
 In regard to using boilers on Yom Tov or Shabbos see Be’er Moshe 1:44, 8:135, Shemiras Shabbos K’hilchoso 2:7, Rivevos Ephraim 6:265:page 302, Ohr L’tyzion 3:21:footnote 1:page 212, Avnei Yushfei 3:55:2, Shraga Hameir 1:41. The concern would be the fact that when one takes hot water from the tap etc he is making cold water get heated up since cold water enters when the hot water leaves the water heater. Refer to Avnei Yushfei ibid if water cooked in a dud-shemesh common in Eretz Yisroel is considered cooked on Yom Tov or before Yom Tov.
 Based on a discussion with an expert in the field.
 Horav Yisroel Belsky Shlita.
 Horav Yisroel Belsky Shlita.
 Bais Yosef 511, Rama 511:2, Elya Rabbah 2, Mishnah Berurah 12, Aruch Ha’shulchan 5, Kaf Ha’chaim 17, Shemiras Shabbos K’hilchoso 14:8, Natei Gavriel ibid: 31:5.
 Horav Yisroel Belsky Shlita, see Mishnah Berurah 9.
 Refer to Rama, Magen Avraham 511:5, Shulchan Aruch Harav 1, Mishnah Berurah 12, 18, Kaf Ha’chaim 17. Refer to Ohr L’tzyion 2:page 252 if one may give a child a bath if he is very dirty. The Avnei Yushfei 3:55 is lenient.
 Shemiras Shabbos K’hilchoso 14:18. Refer to Biur Halacha 302:10 “d’lo.”
 Refer to Rav Akiva Eiger 326:4, Mishnah Berurah 326:24, Igros Moshe O.C. 4:74:rechitza 3, 4:75:1, Be’er Moshe 6:73, Divrei Chachumim page 152:432, Nishmas Shabbos 5:186, Ohr L’tzyion 3:21:1.
 Horav Yisroel Belsky Shlita, see Shemiras Shabbos K’hilchoso 14:11 who permits it. Refer to Sharei Teshuva 326:1, Biur Halacha 260 “poshrin.”
 Refer to Mishnah Berurah 326:30, Kaf Ha’chaim 326:43, Yabea Omer O.C. 4:28, see 4:27:4-7, Ohr L’tzyion 2:35:5, Shoneh Halachos 326:15.
 Ben Ish Chai Yisro 2:15, Aruch Ha’shulchan 326:11, Kaf Ha’chaim 326:43, Shemiras Shabbos K’hilchoso 14:16, Nishmas Shabbos 4:374, 5:191, Migdal Dovid 3:page 679-680, Bris Olom memachek 5, Oz Nedberu 10:16, Yabea Omer ibid, Yalkut Yosef 326:12:footntote 23, Meor Ha’SHabbos 1:page 534 quoting the opinion of Horav Fisher zt”l, Shalmei Yehuda 9:page 158:footnote 9 in depth, Be’er Moshe 5:87:pages 133-136, 8:247, Kitzur Hilchos Shabbos 32:4:footnote 17, Ohr L’tzyion 2:35:5, Am Mekadshei She’vei 2:page 13:footnote 31, Kovetz Teshuvos 1:27:2. The Shabbos Kitchen 19:footnote 17 says Horav Moshe Feinstein zt”l would admit liquid soap is permitted to use if one would mix the soap with water. Refer to Chut Shuni 1:page 117. (Horav Yisroel Belsky Shlita maintains that one should mix the water with the soap before Shabbos and then using the soap is permitted). This is the eitzah of the Aruch Ha’shulchan 321:11, 326:12. See Kaf Ha’chaim 326:43, Shemiras Shabbos K’hilchoso 14:16, Oz Nedberu 10:16.
 Igros Moshe O.C. 1:113. Refer to Be’er Moshe 6:132, Shraga Hameir 1:28.
 Shemiras Shabbos K’hilchoso 14:13, Chut Shuni Shabbos 2:page 62.
 Refer to Mesechtas Shabbos 128b, Rambam Hilchos Shabbos 9:11, Sefer Chasidim 827:page 479. Refer to Pri Megadim M.Z. 326, Chai Adom Shabbos 22:12, Yalkut Yosef 326:page 60, Shevet Ha’Levi 1:97:3, 5:45. See Taharas Ha’Shabbos K’hilchoso 2:pages 312-372 in depth on this inyun.
 Shemiras Shabbos K’hilchoso 14:20:footnote 64, Nishmas Shabbos to 320:260, Ohr L’tyzion 2:35:2, Am Mekadshei She’vei 2:page 106:footnote 22.
 Rambam Hilchos Shabbos 9:11, Magen Avraham 320:23, Shulchan Aruch Harav 326:6. Refer to Minchas Shabbos 86:6.
 Ben Ish Chai Pekudei 2:8, Kaf Ha’chaim 320:111, 326:31. Refer to Orchos Shabbos 13:footnote 89 quoting Horav Elyashiv Shlita as permitting this as well. The Eishel Avraham Butchatch 320:17 also permits it because it is a pesik reisha d’lo nicha ley. Refer to Be’er Moshe 1:31 who say one can not be lenient even in the above situation.
 Ben Ish Chai ibid, Am Mekadshei She’vei 2:page 106:footnote 25, Nishmas Shabbos 5:188. Refer to Oz Nedberu 1:55.
 Oz Nedberu 1:55. Refer to Yesodo Yeshurin 5:page 153. One may not dry his hair with an electric dryer that was left one from before Shabbos (Rivevos Ephraim 6:192:3, Yalkut Yosef 4:page 65).
 Emek Teshuva 1:36.
 Shemiras Shabbos K’hilchoso 14:21:footnote 68.
 Ketzos Ha’shulchan 33:badi 8, Emek Teshuva 1:36, Opinions of Horav Chaim Kanievesky Shlita and Horav Korelitz Shlita quoted in Am Mekadshei She’vei 2:page 106:23. Refer to Shulchan Shlomo 326:9:10:2.