Bathing on Shabbos
2. [The following rules apply to] hot springs, for example, the springs of
Tiberias: If they are in a pool in the ground, in a place that is not
covered by a roof, one may bathe one's entire body in them on Shabbos
(1). If, however, they are in a container (2) or in a roofed building (3),
it is forbidden to bathe in them. Indeed, it is forbidden even to enter
the room to sweat. There are opinions, however, which maintain that, as
long as they are in a pool in the ground, one may bathe in them even if
they are in a roofed building (4).
(1) Only water heated by fire is prohibited.
(2) People may mistakenly assume that the water in the container was heated
by fire, and conclude that it is permitted to bathe on Shabbos in water
heated by fire.
(3) A roofed room could be used for sweating, which, as we saw in HY 86:1,
is also rabbinically prohibited on Shabbos, even if the source of the heat
is a natural hot spring; consequently, the sages prohibited bathing in a
roofed hot spring.
(4) According to this lenient opinion, the sages only prohibited sweating
in places where it would be prohibited to bathe. Therefore, it would be
permitted to bathe in a hot spring situated in a roofed room. The Mishna
Berura (326:11) rules that one may rely on this opinion.