It is forbidden to heat water on Shabbat to the point that one could be scalded. There are different estimates of how hot that is, but scientifically, that is between 130 and 140 degrees Fahrenheit (54-60 C). One may not wash in water that has been heated on Shabbat, or even drink it. If it was heated before Shabbat, one may only wash their hands, face and feet with it; not the entire body. What about the hot water in my sink or shower? Logically, it should be forbidden. The water in the boiler was heated before Shabbat, and should then be suitable for the limited washing mentioned above. But once a sensor "notices" that the water level has dropped, cold water enters the boiler to be heated. I am therefore "cooking". Besides this, when the water temperature falls below a certain point, a flame goes on to reheat the boiler. Perhaps I caused that to happen by removing some hot water, thus allowing in the cold. In light of all of this, most Ashkenazim will not touch the hot water faucet on Shabbat at all. Many Sepharadim have also accepted this approach. However, a very distinguished minority of Sepharadic rabbis has challenged this. Firstly, it is a long accepted practice to heat a mikveh, even on Shabbat. How? No one is heating it! A thermostat controls the temperature automatically. Any human influence on that process is a very indirect causation. Rav Moshe Feinstein suggested that the person entering such a mikveh should have in mind not to enjoy the heat. Sepharadic rabbis generally scoff at such suggestions. Therefore, the "distinguished minority" I spoke of permits, especially in cases of illness or great discomfort, the use of hot water for bathing, showering, or any other similar usage. (I am speaking of a hot water system employing a boiler. The type of system where water is heated instantly, as it comes out of the cold water pipe, being more direct, is much more problematic.) Rav Yitzchak Abadi goes a step further. If the thermostat is set for well under 130 degrees, the water never actually "cooks", and may be used freely, even where no minor emergency exists. Therefore, many Sepharadic Jews freely shower on Shabbat morning, and see it as part of the mitzvah of honoring the Shabbat. This is also my practice. Although some object to the washing of hair on Shabbat, even most Ashkenazim permit it if patted dry with a towel, rather than being wrong out. Many Sepharadim are far more lenient, understanding the admonition found in some sources that one may not wash hair as referring to a hair garment, not hair still attached to a living person. As mentioned earlier, Sepharadim have no problem with solid soap. The ways of the Torah are "ways of pleasantness".Enjoy your Shabbat.