Board fan of the ender 3 does not turn on till 2nd layer because its coupled with the filament cooling fan, I do a lot of large prints and don't feel comfortable having the fan off for so long.

so I designed a easy fix

Needed: 50x50x15 blower fan (WINSINN 50mm Blower Fan 5V 5015) on Amazon they have them with USB so no wiring needed, fan is held in place with hot glue, I guess you can do it prettier than me :-)

I attached rubber bumpers to keep it from moving, double sided tape works too, make sure wires are out of the way, https://www.thingiverse.com/thing:2880021 clips works best.

Make sure the fan pulls the air out of the Board case as the build in fan pulls air into the board case.

UPDATE:
After using the fan for a while I noticed warping on larger Prints only on the right corner, and I guess it might be the fact that the fan blows air right under the Bed and cools the right corner down. I might have to add a duct to divert the air to the side or front.

Creality

Ender 3

0.28

30

Holes print fine without supports