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I've heard that there is a dispute among halachic decisors if one is allowed to use an urn like the one shown below on Shabbos

hot water urn with external tube to show the amount of water left

because the outside water tube gets cold over Shabbos but then gets reheated in the cup. Both sides agree that "Yesh bishul achar bishul" (there is cooking after cooking).

What is the argument? It seems to be a machlokes b'mtzius (an argument over a fact).

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Are you talking about the particular urn, because there is a tube containing water that shows the fill level, or are you talking about any urn, because of concern that some water might be left in the spout? – Seth J Jan 10 '12 at 3:44
@SethJ the one that shows the water level. – Shmuel Brin Jan 10 '12 at 3:45
Nothing at all firm (which is why I'm not posting this as an answer), but an idea: There are, I believe, some who hold that if a few drops are left in the bottom of a cup then it can nonetheless be refilled with hot water. Perhaps whatever argument applies there applies here, too? – msh210 Jan 10 '12 at 5:13
I imagine this is an argument about weather or not the water in the level guage actually gets replaced after a cup is filled or not and if it's replaced, if it's "completely" replaced. I think this is an argument about halacha not fact, because of what msh210 mentioned. But I'm not confident enough to post this as an answer. – avi Jan 10 '12 at 9:05
@ShmuelBrin could you name one or two halachic decisors from each side? – Y ez Jun 15 '15 at 18:35

Rav Yechiel Perr told me that that after speaking to scientists he concluded that to use such an urn is not a problem as the water in the tube has already exchanged from the water inside.

The sefer Orchos Shabbos (Chelek 1: Perek 2: Seif 30) rules this as well. See note 47.

Rav Ben Zion Abba Shaul in Shu"t Ohr Letzion (Chelek 2 Perek 30 note 10) rules that even according to those that hold yesh bishul achar bishul this urn us not problematic.

This is not to be relied upon for practical halacha CYLOR

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Orchos Shabbos writes the same (and I opened this thread to post it as an answer until I saw your post here) – Y ez Jun 15 '15 at 18:37
@yEz shkoyach. Where in orchos shabbos? May I incorporate that into my answer? – Mefaresh Jun 15 '15 at 18:39
That's what comments are for! He has a whole section about hilchos urns in his bishul achar bishul section. Chelek 1 ch. 2 intro to se'if 27 os 5 – Y ez Jun 15 '15 at 18:53
Oops when I said os 5 I meant 3 (or gimmel). There, and in note 42, he explicitly makes the "scientific" point. – Y ez Jun 15 '15 at 19:05
Thank I see now. – Mefaresh Jun 15 '15 at 19:08

As Rabbi Willig explained it, one factor would be whether or not we are concerned with heating more water already above yad soledet bo. Another is if this water's temperature is in fact below that (say this water is at 140 F...is that above or below). There may well be other factors. Tipping the urn backwards before Shabbat to ensure all water is pre-boiled seemingly does help, and was in fact his advice in this case.

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