2015 MacBook Air fan constantly turns on after upgrading to OS X El Capitan
Check whether any applications are constantly running and burning CPU by using Activity Monitor (Located in /Applications/Utilities).

If there are no applications burning CPU, try resetting the SMC:


Shut down the computer.
Plug in the MagSafe or USB-C power adapter to a power source and to your computer.
On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
Release all the keys and the power button at the same time.
Press the power button to turn on the computer.


Source: https://support.apple.com/en-us/HT201295