Is there a problem of "Rechitzat Kol HaGuf" on Shabbat? Additionaly, what if I turn the boiler off before Shabbat - can I then take a hot shower?

The dry halacha is that it is not allowed to bathe (or shower) the entire body in hot water on Shabbat, even if the water was heated before Shabbat. One may wash parts of the body, but not the entire body, even not at the same time. One may bathe in cold water, providing one does not wash long hair which could be "squeezed". The Biur Halacha permits washing for a person who will have great physical distress, even if the person is not ill. So if a person is accustomed to bathing daily and would be in great distress missing a day, bathing could be permitted. This is with the proviso that the problem could not be solved by washing parts of the body, or by washing in cold water. It must also be in a situation where running hot water will not cause cold water to flow into the boiler and become hot. That problem, of course, is independent of the bathing issues.