Thank you very much for your replay. I have to admit that I didn´t understand that a dweller had to operate the vent. I should have read the manual more carefully. ;-)
Now I have a nice working bunker. But I am stuck somehow as I cannot develop further or dig deeper.
Besides I found a bug. If you build storage after storage the mineral bar display will increase. If you continue building after it has reached it´s final size you will trigger the bug and the bar is broken.
Thanks again for that wonderful game.