Question: We almost finished all the water in the electric hot-water urn. We would like to add more water to boil it up. Is that permitted on Yom Tov?
Discussion: Generally speaking, it is permitted to boil water for drinking purposes on Yom Tov. However, electric urns run on a thermostat which turns on the electric heating element as soon as water goes below a certain temperature. Therefore, pouring large amounts of cold water into an electric urn would essentially be turning on electricity, which is prohibited even on Yom Tov.58
Question: My urn has a Shabbos mode. May I refill it on Yom Tov?
Discussion: The term Shabbos mode (or Yom Tov mode59) can mean many things. Sometimes it simply means that the buttons that dispense water electronically are deactivated, but the thermostat is still on, determining when to turn the heating element on and off. In this case the halachah does not change. If it can be verified that the thermostat is actually deactivated in the Shabbos or Yom Tov mode, then it would be permitted to add cold water and wait until the water heats up.
Question: Practically speaking, is there a permissible way to add water to an urn on Yom Tov?
Discussion: There are two ways that it may be permitted:60
Adding a small amount of cold water while the urn is still full and hot. In this case it is not inevitable that the thermostat will turn on the heating element.
One may boil water in some other pot on the stovetop, and then add the hot water to the urn to keep it warm.
58 Yeshurun XXVI, page 583. Mishneh Halachos V:68 suggests reasons to permit this, but recommends that one act stringently.
59 Tests conducted by the Star-K on electric urns have shown that even on Yom Tov mode, the thermostat is not necessarily neutralized.
60 See Yeshurun ibid.