Q. The hot water pot that we use for Shabbos has a water level indicator. I read that this may be a problem since some of the water in the indicator flows into the pot (and is heated by the rest of the water) whenever water comes out of the pot. How can I know if I'm allowed to use it or not?
A. Shabbos hot water urns often have a transparent tube water level indicator on the outside to check how much water is still available. Since when water is taken from the urn, some water from the tube goes back into the canister, and that water may be relatively cold it will be reheated again.
Since each time one removes some water from the urn some water leaves the indicator and mixes with the hot water inside the urn. Presumably, the water in the indicator is at a cooler temperature than the rest of the water. As such, by removing water from the urn, one indirectly causes some water to be cooked.
Yehaveh Da’as (6: 21) is lenient based on the combination of a number of factors as follows. Firstly, we may reasonably assume that the water in the indicator tube had, at one point, been heated. Even though it cools in the tube, it had, in all likelihood, been heated together with the other water in the urn at some point. According to some Poskim, one may reheat water on Shabbos if it had been heated before. Although Halacha does not follow this view, and forbids reheating on Shabbat water that had cooled, yet the lenient position may nevertheless be taken into account together with other considerations. Such as the one taking water from the urn, has no intention to cook the water in the indicator tube that is then transferred into the urn, nor is he interested in doing so. According to one opinion (the view of the Aruch), one is allowed to perform an action that will inevitably result in a forbidden action if one has no interest in that result ("Pesik Resheh De’lo Ichpat Leh").
The accepted Halacha does not follow this opinion; we follow the view that the Sages forbade performing an action that will inevitably result in a Melacha (forbidden action), even though one has no interest in the result. Nevertheless, we may take the lenient position into account in determining the status of using an urn with an indicator. Prohibiting the use of such an urn depends upon two questionable assumptions: that reheating water is forbidden, and that unintentionally causing a Melocho is forbidden. Even though the accepted Halacha follows the stringent view on both issues, nevertheless, since this situation entails two uncertainties, we may permit the use of such an urn on Shabbat.
Of course, it would be advisable to avoid this question and purchase an urn without an indicator. However, Horav Ovadyah Yosef maintains that those who use these urns on Shabbos have on what to rely and may continue using them.
Rav Ben Zion Abba Shaul (Ohr L’tzion 2:30:note 10) is also lenient. (excerpts from Daily Halacha)
STAR-K consulted with engineers and undertook the testing of urns to see if the water moves from the tank into the tube, and the amount of time it would take to do so. Different models of urns were heated, then a food dye was poured into the urns. In all cases, the dye began flowing into the tube almost immediately. After a very short time, the tube was the same color as the water in the urn. This indicates that there is an exchange of hot water into the tube.
Even if the water in the tube does not remain boiling hot (especially at the top of the tube), the Ramo (O.H. 318: 15) holds that water which has once been heated and has not totally cooled down is no longer subject to restrictions upon further heating.
Horav Shlomo Miller's Shli'a opinion is also to be lenient.
Rabbi A. Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit'a
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