in response to the question ” Can I plug my Shabbat urn into a timer so that it will go off late Friday night and come on again on Shabbat. The water will be fully boiled before Shabbat “. Which was answered on the 20th November:
The Rav wries their is no problem of bishul. Water is a liquid so we hold yeish bishul acher bishul. How is this not a problem of bishul?
Bishul is not the issue here because no one is doing any action on shabbos that is going to cook the water, even after it cools down. :