if someone forgot to plug the hot water urn before shabbos and they realize it about 10 minutes before shkiah can they then plug it in even though the water will boil up when shabbos begins
According to most poskim, if the urn doesn’t have adjustable setting it should be permitted, because there is no fear that one might raise the heat to sped up its cooking. However one must be careful not take any water out of the urn until the water is fully cooked. R’ Moshe Feinstein zt”l however held that will it is not permitted unless the water reaches 1600F before sunset. This is because he was afraid that one may come to take water out of the urn before the water is fully cooked, which would speed up the cooking of the rest of the water.
See Halachos of Shabbos pg. 348, Shmiras Shabbos K’hilchoso 1 ftnt.42 (old edition), Igros Moshe O:CH 4-74 (Bishul 3).