Thx a lot for the help and sorry for the delay in answering. I have installed a little app that shows the temperature and it's never hot. I guess it's why the fan is not working (or too slowly to notice anyway).
I'm really glad to have bought this machine. I somewhat regret my ipad retina screen but everything else is better in the Air. Pity I have to keep a big Dell to play games, I'd love to gently push Microsoft out of my life.