Many people fill a hot water urn and plug it in before Shabbat, so they can have hot water throughout Shabbat. An interesting question arises concerning the use of urns that have a water level indicator on the side. The indicator is a transparent tube situated outside the canister that contains the hot water. The indicator fills with water, and then, when water is taken from the urn, some water from the tube goes into the canister, thus lowering the level of the water in the tube. This device enables one to check the level of the water in the urn without having to open it and look inside.
Is it permissible to use on Shabbat an urn that has such a mechanism? Each time one removes some water from the urn for his tea, for example, 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. Would this be a violation of the Shabbat prohibition against cooking?
Hacham Ovadia Yosef addresses this question at length in his work Yehaveh Da’at (6:21), and he concludes that one may be lenient and use such an urn on Shabbat. He arrives at this ruling based on the combination of a number of factors. 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 authorities, one may reheat water on Shabbat if it had been heated before Shabbat. Halacha does not follow this view, and forbids reheating on Shabbat water that had cooled, but the lenient position may nevertheless be taken into account together with other considerations. Secondly, when one takes water from the urn, he 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 Melacha 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, those who use these urns on Shabbat have on what to rely and may continue using them.
Summary: It is permissible to use hot water urns on Shabbat, including urns that have a water level indicator on the side. Preferably, one should use an urn without an indicator, but strictly speaking it is permissible to use such an urn.