What issues may be at play when adjusting the temperature of an urn on Shabbat, e.g. from 200° F to 80° F, specifically on an urn pictured below*? (Assuming the water is already pre-boiled before Shabbat and no lights will turn on when temperature is adjusted. Would also appreciate technical input; it greatly helps in determining halachic outcome.)

temperature controlled urn

*One company I am aware of that makes this model is Magic Mill.

  • at the least tape the temperature knob so it cannot be adjusted by accident on Shabbos. – sabbahillel Oct 29 '17 at 4:55
  • @sabba what would be wrong with accidentally adjusting it (is the OP's question) – Double AA Oct 29 '17 at 13:58
  • Another point that applies to Yom Tov would be if it is a continuous rheostat or a click type (on off) type of controller. – sabbahillel Oct 29 '17 at 16:12
  • Many of these types of urns have a built-in thermostat. In some cases, the unit turns on when it senses that the water temp. is too cold and it actually boils the water. I've heard the urn doing this, and seen the steam coming out the top of the urn. I can't say if this specific urn is prone to do this, so you need to check. – DanF Oct 30 '17 at 2:51
  • "Would also appreciate technical input". I think most of the responsibility is on you to find this out. For one thing, we don't even know which brand urn this is, so providing at least this info will focus your question better. – DanF Oct 30 '17 at 2:54

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