Can a regular water urn be used for shabbat assuming the water is put in prior to shabbat or does one need a shabbat mode? What does that do halachicaly?
Before answering about using an urn on Shabbat, we must understand what the issues would be. Chazal enacted a prohibition to put raw food on an open fire right before the onset of Shabbat. They enacted this prohibition to prevent people from mistakenly adjusting the flame in order to speed the cooking process. Adjusting the flame would would be a transgression of both kindling and cooking. To permit placing uncooked food on a fire close to Shabbat, Chazal required meeting one of two conditions. Either the coals must be removed, or the coals must be covered in a way which prevents one from stirring them.
Nowadays, common practice is too place a “blech” on the fire. This is comparable to covering the coals. Some Poskim require that one also cover the knobs on the stove, to prevent them being adjusted.
Putting cold watery in an urn right before Shabbat is like placing raw food on a fire. One must fulfill one if the two aforementioned conditions in order to do so. An urn with only one temperature setting may be used on Shabbat, whether it has a special Shabbat mode or not. This is because there is no way to raise our lower the heat. The fire of the urn is considered covered, this fulfilling the second method of placing raw food, in this case cold water, on a fire right before Shabbat. If the urn has an adjustable temperature, it may be used provided the water was boiled, and is still over 160 degrees Fahrenheit at the onset of shabbos. This would give the water a halachic status of cooked, and negate the issue of placing raw food on a fire in the first place.
Shabbat mode on an urn generally disables the electronic buttons. This prevents one from accidentally using the buttons on Shabbat. It would appear that Shabbat mode has no ramifications in regard to using an urn on Shabbat.