5. Insulating within the Tiberius hot springs:[1]

A. Placing a bottle of water and the like into the spring from before Shabbos:

A channel of water from the hot springs of Tiberius is forbidden to place through it, even from before Shabbos, a tube of cold water which its opening [of the tube] extends past the channel and its [cold] waters spill into a ditch in the ground. This is done in order so the [cold] water that is in the tube heat up upon going through the channel of hot water.

The Reason: Doing the above is considered similar to insulating with material that increases heat being that these [cold] waters [that are in the tube] are being insulated within the channel of the Tiberius hot springs which are considered a material that increases heat, and it is explained in chapter 257 [Halacha 1] that it is forbidden to insulate with material that increases heat even from before Shabbos.

If one transgressed and placed the tube in the spring: If one transgressed and insulated [in material that increases heat] then it is forbidden [to benefit on Shabbos from that food]. Therefore, if one transgressed and placed the tube of cold water through the channel of [hot water] then the waters which are spilling from it into the ditch are forbidden to use either for bathing or for drinking, just as is the law regarding if it were heated on Shabbos in which case it is forbidden to wash in them even one’s face, hands and feet, and even [to wash] only a single limb [is forbidden].   

B. Having cold water flow into the hot spring from before Shabbos:

However, all the above [restrictions] is only with regards to when the actual tube is encompassed from all sides [by the hot water] and it opens outside the channel however if one has the [cold] water poured into the water of the channel then this is not considered insulation at all being that the cold water mixes into the hot water and is not insulated within it. Therefore, it is permitted before Shabbos to have cold water flow into the channel and have the flow continue throughout the entire Shabbos. Doing so does not contain [a prohibition of] cooking because the action was begun from before Shabbos and finishes on its own on Shabbos as was explained in chapter 252 [Halacha 1].

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[1] Admur 326:3