There are a lot of frum stores that sell the Cheman hot water urn that has a shabbos mode. They have many of the same urn under different brand. The shabbos mode disables the electric dispenser button and you have to pump manually to dispense. The shabbos mode also keeps the water at constant temperature and does not re-boil.
The problem that no one seems to be concern with is that there is a tube in the unit for water indication level that mixes with the dispense water when you pump.
One of the main issues with urns (Shemiras shabbos K’ehilchasa) is that cold water from the tube mixes with the hot water and person is oiver on bishul. Back in the days those tubes were on the outside and people used to crack and disable them. This tube is on the inside, yet it’s not part of the heating unit and it seems that there is an issue of the waters mixing.
When I asked a Posek about this, he mentioned that out of limud zchus, it’s possible that since the water is close to the heating unit, that it’s warm and there won’t be issues of bishul, but for sfaradim it would be an issue.
There are many yidden that own such units and if you can please advise and let us know if it is mutar to use these on shabbos.
here is a link to the urn:
I don’t think that it is a problem because there is a law in physics that hot and cold water in two contacting vessels will mix by themselves after an hour or so, and from then on the water is constantly mixing. Therefore if the water in the urn is totally cooked before שבת, this issue is only applicable for a short time after being מקבל שבת, and even if it finishes cooking a little later.
Even for Sefardinm it should also be permitted The reason being that the water in the tube is not going to cool down to the point that it isn’t yad soledes bo, as it is constantly mixing with the hot water. Secondly, even if you will say that it will cool down when the tube is exposed, here it is close to the hot water and it isn’t cooling down. Thirdly the Ohr L’tzion 2-30 ftnt 10 says that even for sefardim it is alright because bishul achar bishul is a machlokes and here even if it would be bishul it is not his intention to do so.
Orchos Shabbos vol.1 pg, 75,and chap. 2- 30 ftnt. 47, that this fact was verified by physicists.