Derivative Sensor Helper - FAQ
To ensure this automation works properly, you need to set up a Derivative Sensor Helper in Home Assistant and install a humidity sensor in the bathroom. In my setup, I use an ENV III Temperature Humidity Air Pressure sensor located on the fan inlet. It’s crucial that your humidity sensor reports changes frequently. I have set my humidity sensor to update every 60 seconds, as this frequency is essential for the blueprint to function correctly.
Note that Home Assistant automatically purges humidity data after a default period of 10 days to prevent data buildup, so there’s no need to worry about excessive data accumulation.
When locating your humidity sensor, it’s advisable to position it as high as possible in the room or as close to the fan inlet as feasible. This ensures the automation can react quickly and effectively to changes in humidity, particularly during showering.
To add derivative sensor helper to HA, follow these steps.
Go to Settings / Devices & Services and click on the “Helpers” tab at the top. Click create helper and select “Derivative”
Fill out the name you would like your derivative sensor to be, select your humidity sensor in “input sensor” (this is my ENV III Temperature Humidity Air Pressure sensor) , precision = 2, time window can be 3 to start but you can change this later (see edit helper below) and in time unit, select “minutes”. Now click “SUBMIT”
Once you’ve created the Derivative Sensor Helper in Home Assistant, return to the blueprint configuration and select or enter that derivative sensor in the “Humidity Derivative Sensor” field. Then, fill out the rest of the settings according to your requirements. For further assistance on setting up the automation correctly, please click here.
Your exhaust fan should automatically come ON now when you have a shower and turn OFF when your finished. 
If you would like to know how to set up the automation settings so if functions correctly Click Here
EDIT HELPER
If you need to edit the Helper. Go back to Settings / Devices & Services / click on the “Helpers” tab / click the helper you would like to edit, in this case “Bathroom Humidity Derivative”. Click on the cogged wheel.
Then select “Derivative Options”
You will then see your settings. Edit as required.
Enjoy
Blacky 
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