809. Warming Food

Shabbos 22:8

If food was cooked or soaked in hot water before Shabbos, one is permitted to soak it in hot water on Shabbos even though it is cold. Even if food is cold and has never before been in hot water, it may be rinsed in hot water on Shabbos so long as doing so does not complete the food’s preparation, but it may not be soaked in hot water for the first time on Shabbos.

Shabbos 22:9

Even though it is not permitted to heat food by using things that are heated the sun, it is permitted to heat them directly in the sun. This is permitted because there is no chance that someone could confuse the sun with fire. One may therefore warm water in the sun. Similarly, it is permitted to put a container of potable water into a pool of foul water in order to cool it. Likewise, one may put a cooked food into a well in order to preserve it.