I read a Q&A in a recent newsletter regarding using an urn on Shabbat and followed the link to the source you provided. I am referring to the following question:
Q: Can the temperature setting be changed on Yom Tov?
A: When the urn is heating (ie the red light shows), one may turn up the temperature to a higher setting. One may not turn it down.
When the urn is not heating (the red light is not showing), one may turn down the temperature setting. One may not turn it up.
I was aware that if a timer is in the “On” position, we can extend the time until it turns off. I was not aware we could do the above-mentioned points. Is the allowance to change the temperature (within the guidance shown) acceptable on Shabbat too or is this allowed only on Yom Tov?
Thank you for your question.
The simple answer is that the heter (allowance) to adjust the temperature setting up or down (depending on the specifics) is limited to yom tov. You are correct about the permissibility of “prolonging the status quo” of a time switch device (so-called Shabbos Clock) even on Shabbos, however I would only approve this (in accordance with the view of Rav SZ Auerbach), in a situation of need, as Rav Moshe Feinstein considers this forbidden due to considerations of muktzeh (ie, the prohibition of moving or adjusting an article whose use is ordinarily forbidden on Shabbat).
One can allow adjusting the temperature knob on yom tov, however, as even fire itself may be manipulated (judiciously) on yom tov.
Thank you again for your sophisticated question.
Rabbi Rashi Simon