most commonly, yet. But you can pipe water that is already heated somehow in it and it will keep it heated.
I mean technically cold water already is heated otherwise it would be 0 Kelvin.
NOW we're getting pedantic!
Here is my cold water heater tank, it uses kerosene and coal to pre-heat the water for my hot water heating tank. This setup is perfect because the gas powered hot water heater makes the electric on demand hot water system at each of my sinks very efficient.
Edit: I used to keep my hot water in a hot water warmer to keep that hot water nice and warm, but for some reason, I could only get warm water out of it.
Theoretically, you can also pipe in boiling hot chocolate, or even semen
not making your hot chocolate with semen
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