By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
Can one put up cold water in an urn immediately before Shkiah on Friday, if forgot to do it earlier? What if one has many guests or a baby that would need hot water during Shabbos?
The water must be at least half-cooked before Shabbos. In the event it was only a third cooked before Shabbos it may be used on Shabbos. However, one may only take water from the urn on Shabbos after it has completed cooked on Shabbos.
This only applies to an urn which possesses a volume knob; where it’s possible to adjust the temperature on the urn. An urn which only has an on and off feature may be filled right before Shabbos. However, one may not remove any water from it on Shabbos until the water has boiled completely.