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Mixing warm and cool water on Shabbat

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Rabbi David Sperling

Tishrei 3, 5781
Question
Hello. If I turn on the cold water from my sink, on Shabbat, and warm water comes out before turning cold, have I broken Bishul? In this scenario, I had no intention of receiving hot water nor of mixing warm water that was heated naturally (summertime) with cold water. Is this breaking Shabbat? What if I knew for a fact that this would happen by turning on the cold water?
Answer
Shalom, Thank you for your question. I assume you are talking about a situation where the water in the cold water pipe gets heated up by the sun. When this happens, when you turn on the cold water in the sink, first there comes out hot water (that was heated by the sun) for a minuet or so, before the water starts running out cold. Firstly, there would only be a question at all if we are dealing with water that heated up to above the level of “yad soledet bo” – that is water to hot to put one’s hand into, above 43-45 degrees. Usually the water that gets warm in the pipe from the sun does not reach this level, and so there is no question of “cooking” in relation to the water. However, if in your situation it does, then we can address the question. Water heated up by the sun itself is permitted on Shabbat (as opposed to water that is heated up by a “fire”, or even by something that was itself heated up by the sun (like a metal pan heated up by the sun, onto which water was placed and heated up – that would be forbidden)). So, the water here is permitted. However, you might have a problem of that hot water mixing with some cold water (that is in the pipe below or above the hot water), and “cooking” it. It sounds unlikely that this would happen in your situation. This is especially so given the fact that you do not have any intent to heat and cook and cold water – the opposite, you desire only cold water. So, by letting the hot water that comes out first run down the drain, you avoid any problems of “bishul”, (See Shmirat Shabbat KeHilchatah (last edition, Hebrew) chapter 1, footnote 131). Blessings.
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