Must one Kosher his water urn to use on Pesach?
Water urns are to be Kashered for Pesach. In the following cases, this may be required from the letter of the law: a) The urn was placed down while hot onto Chametz; or b) The urn was washed in a sink with hot Chametz; or c) One had Chametz soak in the urn for 24 hours; or d) One poured hot water from the urn onto Chametz [such as onto instant noodles, a Chametz soup mix, or he poured the leftover hot water into a Chametz sink]; or e) One heated a Chametz food, such as bread, on top of the urn. If one is certain that none of the above took place with the urn, then from the letter of the law, it is not required to be Koshered. Practically, however, it is difficult to ascertain the above as well as we suspect that perhaps a crumb of Chametz once fell inside, and therefore it is to initially be Koshered in all cases.
How to Kosher: If the urn is made of metal, then it is to be Kashered through Hagalah. If it contains plastic, it is disputed if it can be Kashered, as explained in Halacha 8. In all cases, one must beware to remove the hardened calcium from the urn before Kashering.
 See Even Yisrael 7:21; Piskeiy Teshuvos 451:14; Nitei Gavriel 84:8
 Nonetheless, even in these cases it is subject to dispute, as some Poskim rule we follow majority usage. However, according to the final ruling that we are accustomed to suspect for minority usage, then the above cases become an obligation to Kosher due to the custom. [See Admur 451:26-27]
 See Admur 451:21
 See Admur 451:60 and 72
 See Admur 451:51 regarding Minhag Nashim to not us ethe pot used to Kosher with Iruiy Keli Rishon due to Nitzuk Chibur; See Rama Y.D. 105:3; Darkei Teshuvah 105:96-100
 See Shach Y.D. 105:23; P”M 173 A”A 1 that it possibly transfers 60x since there is liquid inside; Maharsham 5:76 that it only transfers a Kelipa
 See Piskeiy Teshuvos ibid who records a dispute regarding if we suspect that perhaps a crumb of Chametz fell into it, however, from Admur 451:21; 43 and 72 it is implied that there is no room for such suspicions, and only if there is probable cause is there reason to be stringent. However, see Admur 451:46 regarding a grinder, that initially we do suspect for a crumb of Chametz. Vetzaruch Iyun.
 Admur 451:21 regarding the handle of a pot and the same should apply here as well; Admur 451:46 regarding a grinder; See previous footnote
 An electric urn can be Koshered through boiling water inside of it until its very top and then throwing into the boiling water an Even Meluban [red hot stone or slab of metal] which will cause the water to overflow and splash beyond the rims of the pot. [Admur 452:1 and 24]
 See Piskeiy Teshuvos ibid