According to the halacha*, electric urns that were manufactured outside of Israel must be immersed in a mikveh (ritual bath) before being used. This can be accomplished as follows:
(1) In order to do the immersion as briefly as possible, the urn should be filled with water beforehand and then immersed quickly in such a way that the water in the mikveh will come into contact with the water in the urn. Afterwards, the urn should be set aside for several days to dry (in a normal position, not upside down!).
(2) An alternative method (this is the only reasonable possibility for a thermos jug with electronic control): the jug should be placed in a plastic bag with the edges of the bag taped along the rim, such that the water of the mikveh will only come into contact with the inside surface (where the hot water will be). Since the thermos jug is "a vessel within a vessel," it is sufficient to immerse only the innermost surface since the outer surface (of the internal vessel) always remains "hidden."
There is an alternative: Instead of immersing the urn, it is possible to cancel the requirement to immerse it! How? A utensil whose components were made by a Gentile and then assembled by a Jew does not have to be immersed because the components are not considered a "vessel," and since a Jew transforms them into one, the vessel is considered as having been made by a Jew. Thus, if the urn is taken apart and reassembled by a Jewish technician it will no longer be necessary to immerse it in a mikveh.
Important Note: Manufacturers will no longer recognize the warranty of an urn which has been fully immersed in water. This fact should be taken into consideration. (Similarly, if an urn is taken apart by a technician who is not approved by the manufacturer, the warranty will no longer be in effect.)
* Note: There are some who do not require immersing urns and cooking vessels if this might cause them damage. See (in Hebrew): Rabbi Reuven Genzel, Immersion of Electric Vessels, Techumin 27.
דרונט בניית אתרים