The question was asked in Halacha if it is permissible to fill up an electric urn on Friday and plug it in right before Shabbat? In such a scenario the water will not be heated until Shabbat itself. The water in the urn is cold, and if plugged in right before Shabbat the water will then become heated through Shabbat. Would this be permissible or not?
Maran in Shulchan Aruch in siman 254:9 writes [listen to the audio clip for the exact quote] that person may not take a barrel of water and put it into an oven on Erev Shabbat (late Friday afternoon.) The concern of Maran at the time was that a fellow might desire to speed up the heating process and therefore stoke the coals inside the oven on Shabbat. The Gemara calls that ‘Shema Yachte’. So from this can we deduce that since our ovens do not contain coals so therefore it would be permissible? Most urns do not have any controls or means to speed up the heating process. So therefore, Halacha does allow to put cold water into such an electric urn right before Shabbat. This is allowed because you are simply inserting the urn’s plug into an electrical outlet, and then after that there is nothing to do. There is no Gezara (decree) against this because it would be considered Garuf VeKatum, which means the water is contained within an enclosure and it is not possible to interfere with the process and alter the heating progression.
However, it must be pointed out that Rav Moshe Feinstein in his book ‘Igrot Moshe’ in Helek 4 Teshuba 74:23, disagrees and says that it is forbidden to put cold water into an urn right before Shabbat. He explains that on Shabbat one might empty some water out of the urn before it fully heats in order to speed up the heating process of the remaining water. So Rabbi Feinstein makes a Gezara on this and says not to pour cold water into an urn right before Shabbat, and not to turn on the urn right before Shabbat. He says that the water should be heated before Shabbat arrives. Again, he bases his ruling on the fact that one might come to remove some water in order to speed up the heating process.
However, the Poskim do not take the ruling of Rav Moshe Feinstein. Therefore, Halacha Lema'ase, one is permitted to put cold water into an urn before Shabbat, and plug it in right before Shabbat. The urn must not have controls which enable the user to adjust the temperature. It must be a simple urn with just a plug. But for those urns with controls, it would be permissible so long as the controls are either removed or covered.
