Position of air vent in a room ?

[ted1234]

Hi, I have been told that a black stain that has come through the wall in my son's bedroom is a lack of air problem.
Does it make a difference/matter if I put the air vent at high or low level in the room ?
kind regards Ted

 

[foxhole]

Black stain is mold sitting on surface , not coming thru wall, a vent will work at any height.

 

[ted1234]

Thanks "fh", I fitted it at low level in the end.
Thanks again....Ted