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Water heated on Shabbat by a solar boiler may be used freely. Using water heated by an electric boiler is more complicated. If the boiler was turned off before Shabbat, and the water in the tank is not scalding hot, one may turn on the hot water tap on Shabbat. But one may not do so if the boiler is on or the water is scalding, since when the hot water is used, cold water will flow into the boiler tank to replace it. This water will then be cooked by the boiler or by the scalding water.
