..maybe not in an electric water heater but in a gas water heater, the heat that isn't absorbed into the tank ends up leaving through the central tube on the top. Usually along with some popping, hissing, and banging sounds if things get really bad.
For an electric water heater, it's likely more of a maintenance concern, if the elements are running hotter because they are covered in scale, they may fail prematurely. Might have an impact on tank life too, although usually it's an issue of an anode rod reaching the end of its life where the tank rusts out.
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