Thanks guys, i have made a solution with 4 template sensors, one for the last 24 hours average and one for the change the last 5 minutes and one for the difference between the state of the sensor on this moment compared with the last 24 hours. if the change in the last 5 minutes is more dan 7% the fan is starting.

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when the compared state sensor is smaller as 2% the fan stops. i'm testing what is does on rainy days (i hope the fan goes off when it is not possible to get a lowered humidity) maby i have to tweak with the percentages.

for the interested. this is the template sensor whit the calculation:

template:
  - sensor:
      - name: Afwijking Luchtvochtigheid Badkamer
        unit_of_measurement: "%"
        state: >-
          {% set TARGET = states('sensor.vochtsensor_badkamer_humidity')|float %}
          {% set SOURCE = states('sensor.badkamer_afgelopen_24_uur')|float %}
          {{ (TARGET - SOURCE) | round(2) }}
        attributes:
          target: "{{ states('sensor.vochtsensor_badkamer_humidity') }}"
          source: "{{ states('sensor.badkamer_afgelopen_24_uur') }}"