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Q. Can I put cold water into my electric urn on Yom Tov?
A. On Yom Tov it is permitted to cook food; therefore, one may put a kettle onto the fire on Yom Tov to heat up water. But it is forbidden to start a new fire or an electrical device, and, therefore, one may not light a new fire. What about putting cold water into an electric urn that is already plugged in and running? Is that like putting a kettle filled with water onto the fire?
It turns out that most urns (and pump pots) operate with a thermostat which turns the urn’s electric coil on and off depending on how hot the water in the urn is. Most of the time, the coil is off, and only when the water temperature drops a few degrees does the coil go on. That is exactly what happens when water from the tap is added to the urn. The ambient temperature water cools off the water already in the urn, and the thermostat senses this and turns on the urn’s electric coil to heat up the water.
Thus, although the person is adding water to an urn which is plugged in and “on”, in truth, when he adds water he is directly causing the coil to ignite and get hot. After considering different aspects of this issue, Rav Yona Reiss ruled that one may not do this on Yom Tov.
To address this issue, there are companies that market urns to the Jewish community, claiming that theirs are designed in a manner that allows the addition of cold water on Yom Tov. In our investigations of these claims, we found that some had merit and were ingenuously designed to avoid concern, but others were not as effective even if they had special “Shabbos/Yom Tov modes”. For most consumers, it is too difficult to test their urn to determine which category their urn fits into, and we, therefore, recommend that they only add water which was already heated in a pot or urn that they placed onto an existing flame.