The Raspberry Pi PoE HAT fan does not spin at all times when powered on - software controls the fan and it will only start spinning when the Raspberry Pi hits a certain temperature:
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Using the current kernel, the fan activates at a slow speed when it hits 40℃, and then speeds up as the temperature increases, around every 5℃ until the fan is at maximum speed.
- The older kernel activated at 50℃
- Note: The PoE HAT is only intended for use with Raspberry Pi OS, so you may need to look into alternative software control if you're using another operating system such as Ubuntu