I was wondering if it is OK to shower using hot water on Shabbat. The water was warmed before Shabbat, and is kept inside a well insulated tank on the attic, that has no connection to any fresh water and comes done simply by gravity. Is that permitted?

Showering or washing one’s entire body, with water that was heated even in a permissible way before Shabbos, is forbidden during Shabbos However, it is permissible to use water that was heated before Shabbos to wash one’s face, hands or feet, when the warmed water uses gravity to come down and not the water pressure from new water coming in the pipes. The above does not apply to one’s entire body. The reason for this Rabbinical prohibition is called “Gezeras Merchatzo’os.” Meaning, that the people working in the bathhouses used to heat the water on Shabbos in a prohibited manner, but claimed that they heated the water before Shabbos.