In the previous Halachot we have discussed that one may not wash one’s body with hot water on Shabbat. This means that even if the water was heated in a permissible fashion, such as being heated before the onset of Shabbat or being heated on Shabbat by a solar water heater, it is nevertheless forbidden to wash one’s body with this water on Shabbat, for our Sages prohibited this so that the bathhouse attendants would not come to desecrate the Shabbat.
Hagaon Rabbeinu Akiva Eiger (Chapter 326) writes that if one is in great discomfort or suffering, even if one is not actually ill, one may wash one’s entire body with hot water on Shabbat. This means that if one is accustomed to washing one’s entire body every single day and if one does not take a shower on Shabbat, this will cause him great discomfort or if for whatever reason one has perspired profusely on Shabbat and this causes one great discomfort, such a person may wash one’s self with hot water on Shabbat.
Hagaon Rabbeinu Akiva Eiger proceeds to support his opinion based on the words of the Rambam (Chapter 6 of Hilchot Shabbat) where he writes that one may instruct a non-Jew to bring him hot water in order to bathe a small child or an individual who is in great discomfort. He derives from here that our Sages did not enact the prohibition of bathing in hot water on Shabbat regarding one who is in great discomfort. Many other great Acharonim rule likewise.
Indeed, Maran Rabbeinu Ovadia Yosef zt”l discusses this issue in his Chazon Ovadia-Shabbat, Volume 6 (page 88) and brings proofs that the prohibition to bathe in hot water on Shabbat is not as stringent as other rabbinic enactments and thus, in situations of great discomfort or suffering, one may wash one’s entire body with hot water on Shabbat.
Nevertheless, the above only applies to the rabbinic enactment prohibiting washing one’s body with hot water on Shabbat. However, realistically speaking, regarding washing one’s self in baths and showers common nowadays, there is another halachic issue which must be accounted for which is that since every household has a tank filled with boiling water (which was heated before Shabbat as we have explained), by opening the hot water faucet in the house, this causes cold water to enter the tank and immediately be boiled by the hot water that was already in the tank which may pose a concern of the prohibition of cooking on Shabbat. (This is likewise a concern regarding the mixture of hot and cold water in the pipes.)
Although Maran zt”l writes (in his Responsa Yabia Omer, Volume 4, Chapter 34 and Chazon Ovadia-Shabbat, Volume 4, page 405) that when the water in the tank has been heated by a solar water heater there is room to allow using the hot water on Shabbat, for although cold water will then enter the tank, since the hot water in the tank has been heated by the sun, any cold water heated by water heated by the sun is not a Torah prohibition (among several other reasons for leniency), nevertheless, when the water in the tank was heated by an electric boiler, even if the electric boiler is off at the moment, the boiling water is considered an object heated by fire and using such water to cook is a Torah prohibition. Thus, one should be stringent and not use water heated by an electric boiler and only use water heated by a solar water heater.
There is nevertheless room for leniency regarding using water heated by an electric boiler on Shabbat when one knows for certain that the water in the tank is no longer boiling and is merely slightly warm such that there will be no prohibition being transgressed when the cold water then enters the tank.
It is likewise permissible to bathe infants with hot water on Shabbat when the water was heated by a solar water heater or if the water was heated before Shabbat and is now lukewarm.
Following the above discussion, such solar water heaters and electric boilers are the most common form of water heating in Israel. In the United States, however, at least in the New York area, the common form of household water heating is a gas boiler which is a large tank filled with water under which there is a constant flame burning (pilot). There is a thermostat within the apparatus which senses when the heat of the water dips under a certain temperature and then proceeds to ignite a larger flame under the boiler and reboils the water in the tank such that the water in the tank will be hot at all times. Thus, based on the rationale discussed above, one must be stringent and not use hot water at all from such a tank on Shabbat similar to the law regarding an electric boiler. The leniency regarding the electric boiler we have discussed above will not apply to gas boilers with a pilot, since, as we have established, the water in such tanks remains hot at all times and will therefore boil any cold water that enters the tank as opposed to the hot water in electric boilers which, after being turned off, will eventually cool off. Therefore, none of the permissible methods of bathing with hot water on Shabbat will apply to such gas boilers with a pilot, for although the issues of the prohibition to wash one’s self with hot water on Shabbat may have been dealt with, the concern of the prohibition of cooking on Shabbat still exists. For further reference, see Responsa Yabia Omer, ibid, Chapter 35.
Summary: One may not wash one’s entire body with hot water on Shabbat. One who experiences great discomfort as a result of not being able to shower on Shabbat may wash one’s self with water heated by a solar water heater. It is likewise permissible to bathe an infant with water heated by a solar water heater.