By: Halacha Yomit team of OU KOSHER
Can one add water to a hot water urn on Yom Tov if this might cause the indicator light to turn on?
Although cooking is permitted on Yom Tov, one is not permitted to light a new fire. Therefore, one is not permitted to add cold water to an urn, since this might cause the light to turn off or on.
Rav Belsky, zt”l, said that one may not even ask a non-Jew to add cold water to the urn. Although turning on the indicator light is an unintended consequence, and strictly speaking one is permitted to ask a non-Jew to do a permitted activity even if this will cause an unintended consequence that is forbidden (psik reisha), in this case it is not permitted.
Rav Belsky explained that there are two heaters in an electric urn. The larger heater turns on when the urn is filled with cold water. Once the proper temperature is reached, the first heater turns off and a second smaller heater turns on to maintain the temperature. When one adds cold water to an urn, one is not only changing the status of the indicator light, but that person is also turning on the larger heater. Since the intent of adding cold water to the urn is to cook the water, one cannot consider the turning on of the heater to be an unintended consequence.
To appropriately add water to an urn on Yom Tov, one should boil the water on the stove and then pour it into the urn.