Fan speed is in part, dependent upon difference between ambient and programmed temp. If it's 90 degrees Fahrenheit in the cabin and you program it for 67, it'll run on high until it gets closer to the programmed temp.
There's also a thermistor in at least one or two of the vents that monitors the system's output temp.
So for example on a frigid morning, the fan speed isn't supposed to ramp up until the heater output temperature warms up a little. (to prevent it from blowing cold air on you when you first started it on a cold morning)
It'll also run the fan speed on high when set to "Max A/C" or Defrost modes.
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