See question 4073 above. If one is using the water that was warmed before Shabbat, was kept warm in an insulated tank, has no connection to any fresh water and comes down simply by gravity, as described above, and he wants to use it for ‘Tisha Kavim’ purification (he belongs to a group that immerse in a Mikva on Shabbat). Can he do it? Why is it any different than immersing in a Mikva?

Although in principle the above case may seem permissible for people who accustom to immerse during Shabbos, especially if a proper Mikva is not readily available, Horav Shlomo Miller’s Shlit’a and our other Rabbonim are of the opinion that this unusual method of Tisha Kabim purification should be avoided. Not only for its peculiar and uncustomary nature and the many different necessary conditions to avoid desecration of Shabbos that could easily be forgotten, but also for the fact that the water emanating from the insulated tank could still be at the temperature of ‘Yad Soledes” or about 40 or 45 degrees and since it was heated before Shabbos on that container, it would be still considered a “Keli Rishon’ or a first vessel. If the water flowing from it falls on the colder water usually found in the floor of the shower facility, it would be considered as being ‘Mevashel’ or cooking the cold water and one would transgress on a Shabbos prohibition.