I designed and printed an external rear fan housing so I could connect a 3” hose to vent the printer to the outside for odour control. This works very well when the rear fan is on.
I have also added some code to my material settings so that the rear fan is on at 50% speed which works well while printing.
However, when changing material the fan is not on and I can smell it.
Is there a way to have the rear fan on (at low speed) at power up so that it is on all the time?
I had the same problem, but my solution worked very well. Because my machine is in a shed in the garden, and I really don’t care about looks, I went back to the full hose and just let it dangle (like an elephants’ whatsit) through the wall of the shed. I put some scrunched up fine mesh in the bottom to stop creepie crawlies getting in. Not pretty, but it is a great talking point.
I have already done that.
My question is about a setting or firmware hack that will automatically turn on the rear fan when I turn on the printer.
Are there any scripts that are triggered at startup maybe?
I guess you can change fan options on klipper config, if you rooted your pronter
I haven’t. But I did.
Now I feel like I just walked out of a long dark tunnel. 
I will spend some time looking for the best place to make that adjustment.
Thanks!