Because of a variety of possible halakhic issues, many do not bathe or shower on Yom Tov. However, in a case of need, one may bathe in hot water as long as the water was heated in one of the following ways: before Yom Tov, on Yom Tov by the sun, or on Yom Tov using a timer. This is one of the differences between Shabbat and Yom Tov: on Shabbat one may bathe in lukewarm, but not hot, water, while on Yom Tov one may bathe in hot water (see Peninei Halakha: Shabbat 14:8).
Some are stringent, maintaining that there is no difference between Shabbat and Yom Tov, and on Yom Tov too one may bathe only in lukewarm water. Others are even more stringent, maintaining that it is rabbinically prohibited to bathe even in lukewarm water on Yom Tov as well as Shabbat. Some Ashkenazim follow this ruling. Nevertheless, in practice the halakha follows the majority of poskim who are lenient and permit taking a hot bath on Yom Tov. When not being able to bathe will cause serious discomfort, such as on Rosh Ha-shana, when Shabbat runs into Yom Tov, and in ḥutz la-aretz where Yom Tov is two days, it is proper to follow the lenient opinion in order to properly enjoy and honor the festival.
If one has a solar boiler (dud shemesh), he may bathe in water that was heated up on Yom Tov. One who does not have a solar boiler may turn on his electric boiler before Yom Tov. So as not to waste electricity, he may connect the boiler to a timer so that it remains activated only for the amount of time necessary.
Unlike on Shabbat, on Yom Tov the hot water tap may be turned on, even if the water is boiling hot, and even if the heating element is working. This is because on Yom Tov there is no prohibition on cooking. However, one may not turn on an electric boiler on Yom Tov because doing so is considered lighting a fire. As we have already seen, it is forbidden to light a new fire on Yom Tov.
If water was heated before Yom Tov, Rif and Rambam allow one to use it at home to wash his entire body. However, they do not allow doing so in a public bathhouse because of the above enactment. This is the opinion of the majority of Rishonim, including Ramban; Ran; Or Zaru’a; Hagahot Maimoniyot quoting Sefer Ha-Teruma; Raavya; Shibolei Ha-leket; Ri’az; Magid Mishneh; SA 511:2. Water that was heated on Yom Tov using a timer or a solar boiler is considered to have been heated before Yom Tov and may be used for bathing on Yom Tov (SSK 14:3; Ḥazon Ovadia, Yom Tov, p. 41).
In contrast, Tosafot, Rosh, Rid, and Itur maintain that even if water was heated before Yom Tov, one may not wash his whole body in it. Even though most Rishonim are lenient, Ashkenazic practice is to be stringent (Rema 511:2; Eliya Rabba; SAH; MB ad loc. 18). This is also the ruling of later poskim: Piskei Teshuvot 511:7 and Shemirat Yom Tov Ke-hilkhato 15:5. In their opinion, those who are stringent and prohibit using lukewarm water on Shabbat are equally stringent on Yom Tov as well (see Harḥavot to Peninei Halakha: Shabbat 14:8:1). However, we have already seen that according to Tosafot and Rosh, the reason for the prohibition to heat water on Yom Tov for bathing is because it is not considered shaveh le-khol nefesh. Following this understanding, it should be permitted nowadays to heat water on Yom Tov in order to wash the whole body, since most people shower every day or two, rendering bathing shaveh le-khol nefesh. The enactment would be limited to washing in a bathhouse, and would not apply to showering at home. This is the opinion of R. Shlomo Zalman Auerbach (SSK ch. 14 n. 21; Shulḥan Shlomo, p. 198 n. 3). R. Mordechai Eliyahu permits a hot shower but not a hot bath (Ma’amar Mordechai, p. 143). I ruled above in accordance with the lenient view, as that is the position of most Rishonim. Even according to Ashkenazic custom, it makes sense to be lenient, since the disagreement is on a rabbinic level and we are generally lenient in such disputes. On the other hand, I did not write that all Sephardim are lenient, because Or Le-Tziyon 3:21:1 permits bathing on Yom Tov only when the water was heated up before Yom Tov, but does not allow the use of water which was heated up on Yom Tov, whether by solar or electric boiler.
. On Shabbat, turning on the hot water tap is forbidden, both when there is boiling hot water in the tank as well as when a heating element is boiling the water, because of the prohibition of Bishul (Peninei Halakha: Shabbat 10:23-24). In contrast, on Yom Tov one may turn on the hot water tap even when the heating element is working and will heat additional water which is not needed. Since it is impossible to use the hot water without having cold water enter the tank, and one needs the hot water, the whole process is deemed a Yom Tov need. Besides, it is possible that the additional water is considered to be heated up indirectly (grama), which is technically permitted (section 5 and n. 6 above). Moreover, there is no prohibition on Yom Tov of heating more water than necessary (as this is a case of marbeh be-shi’urim – we see this in SA 503:2 and SSK ch. 2 n. 22, which disagree with Shevut Yitzḥak, vol. 6, p. 94). In any case, there is no Torah prohibition involved, since guests might show up unexpectedly and make use of the hot water.