Most fans can be mounted either horizontally or vertically. I've done both.
And I've seen plenty in the skylight well. I installed one in my parents home in 1974 and it worked out just fine.
Need some feedback about installing an exhaust fan in a skylight.
The large skylight is shared directly above two shower stalls and so I need something that will rapidly take the moisture away from the light well. Our HRV vent that's mounted in the skylight now is not sufficient to rapidly take away the moisture. I can buy a Nutone/Broan humidity sensing fan (QTREN100H) and mount it on the skylight wall but the manufacturer specifically says it's for flat ceiling mounting only.
As a potential workaround, my questions are:
a) has anyone here successfully mounted one of those Nutone's in a skylight wall?
b) would damage occur to the bearings if mounted on vertical axis?
While small motors like this with bushings for bearings are optimally operated vertical, that is not the reason for the wall prohibition. Hot humid air goes straight up to the ceiling. The install instructions call for mounting on a flat ceiling up to 12 feet high, for proper operation of the humidity sensor. If you need to mount it on the wall, you could install it for timer operation, rather than sensor operation.