Here's a blurb from the FAQ section of this site.

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Debbie, here's what the FAQ section of the BV site states with
regards to ceiling fans and vaulted ceilings.

What does it take to put up a ceiling fan on a vaulted
ceiling?
Depending if you already have a fixture or not. If you do your
wiring is already there. But, I would bet that the box does not
meet NEC code. A normal ceiling box will not properly support
the fan and the torque of the motor. They sell a kit so you can
remove the existing box and replace it with an approved box. Try
your home center or electrical contractor. If you don't already
have a fixture present, you'll have to install an approved box and
wiring. If you're not familiar with this type of work, call a skilled
contractor. Ceiling fan manufacturers sell a variety of
accessories for sloped and vaulted ceilings.