Using Hot Water Faucet on Shabbos
Question
Dear Rabbi! Is it permissible on Shabbat to bathe with hot water from the hot water faucet after the boiler has been turned off? I should note that there is no solar heater, as I do not do anything with the hot water usage.
Answer
Greetings,
There are several prohibitions in this, specifically:
A. The act of turning on the hot water faucet itself is prohibited because it introduces new water into the boiler, and since the boiler is very hot, the new water that enters it becomes cooked upon contact with the boiling-hot water.
B. There is a prohibition against bathing with hot water on Shabbat, even if it was heated before Shabbat.
C. It is customary not to bathe at all on Shabbat, as there are several prohibitions involved.
Source
Shulchan Aruch (Orach Chayim, Chapter 318, Paragraph 11).