What is the halacha with regards to using a hot water bottle on Shabbos?

            A hot water bottle is used either to warm one’s bed in the winter or is placed on the abdomen to ease stomach pain. There does not seem to be any halachic reason why one should not be able to use it to warm one’s bed. However, if we transport ourselves to a time when there were no pre-manufactured hot-water bottles and people would use an open vessel and place it on their stomach, we will understand what the Shulchan Aruch is referring to.

The Mechaber says [1] that it is forbidden to place a k’li with hot water on one’s stomach even during the week. The reason is because the water might be boiling hot and endanger the person. Rashi adds [2] that on Shabbos it is doubly ossur because the water might spill on one’s body resulting in a person bathing on Shabbos in hot water. We are referring to water heated on Shabbos and therefore one may not wash even a small portion of one’s body with this water. [3]

            Is there a problem using a closed k’li?

            We find a machlokes Rishonim. According to Rashi who holds that the problem is that we are concerned lest the water spills on one’s body, when the k’li is closed, like contemporary hot water bottles, there is no problem. According to Tosefos who holds that one may not place a k’li with hot water on one’s abdomen because it is considered using medication, even a closed bottle is prohibited. Accordingly, when one’s intention is merely to warm one’s bed, one may use a closed water bottle.

            Does that mean that it is forbidden to ease a stomach ache with a hot water bottle?

            If a person is classified as ill, which means that he is either bedridden, [4] or his entire body aches, [5] he may use a hot water bottle. If one is not classified as ill, one may not use any medication, and since placing a HWB on one’s stomach is a type of medication one may not use it. However, in the winter months, where it is common that one places a HWB in one’s bed for warmth, one may do the same when one has a stomach ache. This is based on a rule which says that one may administer medication when it is something that healthy people do as well. [6] For example, healthy people drink brandy, therefore one who has a sore throat or a toothache may drink brandy in the normal manner, even though one’s intention is to ease the sore throat. One may not gargle with the brandy because then it becomes noticeable that one’s intention is for medicinal purposes.

Hence, in the winter months one may place a HWB in one’s bed even when one’s intention is medicinal because healthy people do so as well. [7]

One may nevertheless heat a towel and place it on one’s stomach, as that is not something associated with medicine. [8]

            Is one permitted to immerse oneself in a mikveh on Shabbos?

            The Mechaber says that one may immerse in a mikveh on Shabbos. This heter dispels two problems. The main problem is that although we rule that it is forbidden to immerse a tameh vessel in a mikveh on Shabbos, [9] nevertheless a person may immerse himself in a mikveh. [10] The second problem is that although our custom is not to bathe even in cold water on Shabbos, immersion is permitted.

According to many poskim, men [11] who immerse on Shabbos morning should try and avoid the hot water mikveh. This is because bathing in hot water is a rabbinical prohibition, and although we find poskim [12] who permitted it and made a distinction between bathing and immersing, yet we see that the Mishna Berura Siman 326:7 says that the mikveh should only be lukewarm (and colder) but not hot. [13]

            May one make use of a sauna on Shabbos?

            Subsequent to the g’zeira prohibiting hot bathing on Shabbos, Chazal prohibited the use of a sauna on Shabbos. This was because people would still bathe after the institution of the g’zeira and when caught ‘red-handed’ they would say that they were merely using the sauna.

            What about the use of a sauna after Shabbos which was heated during Shabbos?

            This question was more pertinent in earlier times, when heating a sauna or bathhouse meant adding wood or coals to a fire and stoking it. We will nevertheless discuss it because this halacha has many ramifications to our current lives.

For the sauna to be hot straight after Shabbos, the bathhouse attendant would have to heat the sauna or baths on Shabbos. Since Chazal prohibited deriving benefit from actions performed by gentiles on Shabbos when performed for Jews, it is imperative to determine whether indeed it was done for Jews.

The Shulchan Aruch [14] teaches us that when the majority of the bathers are gentiles, we regard the heating as being done solely for them and Jews may bathe or sweat in the sauna straight after Shabbos. If the majority are Jews, or Jews and gentiles equally, we regard the heating as being done for the Jews and they may not bathe straight after Shabbos. In such a case they must wait the time of bichdei she’ya’asu, i.e. the time it takes to do the prohibited melacha. In this case, if for example it takes 3 hours to heat the sauna or bathhouse, a Jew must wait 3 hours before entering.

            Where is this applicable today?

This applies to any action a gentile performs on Shabbos and is used by Jews after Shabbos. If it takes 2 hours to open and heat up a swimming pool that is used mainly by Jews (separate swimming), and the caretaker makes the preparations on Shabbos, the bathers must wait 2 hours after Shabbos before swimming.