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Note: The culturing flask isn't designed to keep the heat like other thermal flasks. It will turn to room temperature in just a few hours. The flask is used to help retain the warmth in the flask for fermentation. The lid outer is made of stainless steel but the internal is made of plastic with a silicone ring so this is why you may feel the heat from the boiling water instantly.
The boiling water is used to preheat the flask for fermentation. The instructions state to heat again with boiling water every 12 hours, however, this could differ depending on ambient temperatures during winter and summer. A couple of hours between heating might be better suited during winter.
Do not surround the glass jar with boiling water - you may kill the culture. Only use boiling water to heat the flask.