We have an instahot water dispenser that dispenses hot water. It basically has a tank that holds water and is plugged in. When water is dispensed new water flows into the tank.
The question is could this be used on Shabbat if the water supply was disconnected? The water is dispensed without the use of electricity (i.e. if you unplugged the unit you could still dispense the water). As long as no new water flows in does this not function like an earn does?
If no water will enter the urn and get hot then it is permitted on shabbos. Even though the thermostat that controls the temperature will possibly activate, it isn’t for sure and it is indirect, and therefore permitted.
There is one other concern, if the urn’s spout in on the bottom, and all the water can run out of it, and the urn will get ruined. This will possibly cause one to come to fix it on shabbos, and then it shouldn’t be used. If the spout is on the side and there will always be water left over in the urn this will not be a concern as long as the urn isn’t tilted, to empty it of all its water.
Shmiras Shabbos Khilchoso1-46, ftnt. 138 from R’ S.Z. Auerbach zt”l