Most people set up an electric hot water urn before Shabbat. These urns are not considered Garuf V’Katum, and have the status of an open flame. This poses the question of how to avoid the prohibition of Shehiya when leaving hot water for Shabbat. The Hachamim were concerned that one might adjust the flame to hasten the cooking process. Under what conditions can one heat water left on such a heating element?
One might claim that the case of water is more lenient, since it could be considered Mistamek V’Ra Lo, a food that gets worse as it cooks. Such a food is permitted to be left on an open fire since a person does not have an interest to advance its cooking. Water could possibly be considered Mistamek V’Ra Lo because it evaporates and there is less water for consumption. Nevertheless, Shulhan Aruch does not hold like that. On the contrary, Shulhan Aruch holds that water is enhanced as it is heated. Accordingly, water at room temperature would not be considered half-cooked, which would have been sufficient, according to some opinions, for food to be left on an open flame. It is more severe than half-cooked, because water is better when it's warmed, and therefore there is still a gezeira that one might adjust the heat.
The Halacha says that so long as the water was heated before Shabbat, it will be permitted to continue heating it in the electric urn. What degree of heat must be reached before Shabbat? Assuming that we hold like Hanania, the food must be at least half-cooked to be left on an open fire. The Menuhat Ahava (exposition of the laws of Shabbat by Rabbi Moshe Halevi (1961-2001) of Bnei-Brak) ruled that it is sufficient to heat the water to 25 degrees Celsius in order to leave it for Shabbat. Other Poskim rule that the water must reach a temperature of Yad Soledet Bo, the point where it begins to create vapor.
The practical outcome of this discussion is that one must turn on the urn in sufficient time before Shabbat so that the water can reach the proper level of heat. Generally, if the urn is plugged in by candle lighting time, which is 18 minutes before sunset, it will get hot enough by sunset to satisfy all the opinions. Even 5-10 minutes will allow the water to reach 25 degrees Celsius, depending on the type of urn. In the event that one did not turn the urn on until immediately prior to sunset, it will be a problem of Shehiya.
The same would be true of putting a kettle of water on an open fire. As long as it reached 25 degrees Celsius before Shabbat, it can remain on the fire. Of course, if one puts the kettle on a blech, there is no issue of Shehiya whatsoever.
SUMMARY
1. One must turn on the Shabbat urn in sufficient time so that the water reaches at least 25 degrees Celsius before Shabbat.
2. If one leaves a kettle of water on an open flame, it must reach a temperature of 25 degrees Celsius before Shabbat.
