"Why should kashrut approval be needed for an electric urn used to heat water?" As it happens, there are several problems with heating water on Shabbat that must be taken into consideration for an electric urn.
The main problems with electric water heaters, thermos jugs and urns, are:
(1) Removing water might hasten electrical activity;
(2) Removing hot water might cause cooler water in the level gauge might to be boiled;
(3) The operation of secondary systems.
(1) Removing water might hasten electrical activity. In order to determine in which urns removal of water might cause electrical activity, it is necessary to understand the different operation modes of various types of urn. The basic technology used consists of two stages: boiling the water (before Shabbat begins) and keeping it hot (during Shabbat). The changeover from one mode to the other depends on the specific type of equipment.
(a) A heating element to keep the water hot: The earliest type of urn, as in the accompanying picture, has two heating elements. One boils the water while the other one, at lower intensity, maintains the high temperature of the water. Such equipment has no thermostat (except for a safety switch to turn off the heating element if the urn becomes empty), and removing water from it will not affect the heating element.
Note that while in most urns and thermos jugs there is indeed a separate heating element for initially boiling the water, there are some models with only one element. In this case, an electronic "dimmer" reduces the power once the water has reached the desired high temperature. From the halachic point of view, there is no difference between these two models. However, some models with a dimmer have an external dial for setting the water temperature. This must be done before Shabbat: the dial must not be adjusted on Shabbat!
(b) Using a thermostat: In newer models (mostly in the case of an electrical thermos jug), the change of status from boiling to maintaining a constant temperature is automatic, using a pressure gauge. If the water cools down (a status of "renewed boiling"), a thermostat causes the equipment to reheat the water.
When water is removed a small amount of heat is lost, and therefore some people will not use such a heater on Shabbat out of fear that this will cause the heating element to be turned on sooner than would happen otherwise. On the other hand, there are those who permit using such equipment, since the effect that a user has on the operation is very small, indirect, and not noticeable. How long it takes for the system to enter a status of "maintaining the temperature" depends on the size of the heating element. (In models that have a powerful heating element, the equipment will never enter a status of "renewed boiling" unless cold water is added.)
The Zomet Institute approves using such equipment only after checking it thoroughly to make sure that removing water does not have a direct effect on the system. Such equipment is given a "technological kashrut approval." This is not considered mehadrin (stringent) approval!
(c) A "simmerstat": In some equipment, the temperature is maintained on Shabbat using a simmerstat, a device which turns the heating element on and off according to a fixed schedule, without measuring the water temperature at all. Such models have an external dial that can be set by the user to change the relative times of heating as a way of fixing the water temperature. This mechanism is given mehadrin (stringent) approval. It is important to boil the water before Shabbat starts. The temperature dial must not be moved on Shabbat!
(2) Heating the water in the level gauge. Most electric urns have a small tube of water that serves as a level gauge, indicating how much water there is in the urn. This tube is connected to the main tank of the urn through a small opening that restricts the free passage of water. Thus it is possible that the water in the gauge has not boiled, and that when water is removed from the urn the water in the gauge will enter the main chamber and be "cooked." The problem is particularly serious in a thermos jug that uses an electric pump for removing the water, since in this case the stagnant water in the pump is far removed from the heating element. The Zomet Institute gives mehadrin (stringent) approval only to models in which the water in the gauge is always maintained at a temperature where a hand will be scalded. Urns where the water in the gauge can be lower than this temperature are given a "technological kashrut approval." It is recommended that a cup of water be removed from such urns before Shabbat to guarantee that the water in the gauge has boiled before Shabbat (in such equipment all the water passes through the gauge). Many rabbis are lenient with respect to reboiling water that has already been boiled (see Igrot Moshe Orach Chaim volume 4, 74:19; Rabbi Tzvi Tal Harim, "Mevashel" 1). [It should be noted that in most of the models where the water in the gauge is relatively cold the water there is boiled when the urn is first heated. Detailed information about specific models can be found in the list of urns with standard kashrut approval.]
The temperature at which a hand is scalded (such that the water will be considered to have been cooked) is 50 degrees C.
(3) Auxiliary mechanisms. Many thermos jugs have an electric pump for removing the water in addition to the manual pump. In order to receive a mehadrin" approval the manufacturer must provide a Shabbat/weekday switch that disconnects the electric pump on Shabbat. (In models that have a thermostat, the same switch must also disconnect the thermostat on Shabbat.)
For a list (in Hebrew) of models approved as mehadrin
, press here
For the standard models (in Hebrew), ("technological kashrut approval
"), press here
Note that most of the urns in the local market were manufactured abroad and therefore must be immersed in a mikveh (ritual bath) before use. For instructions about immersing the equipment, press here.