Generally they are placed with the vents facing the house, for cosmetic
reasons. With the vents facing the house you see all metal vs all open
space when you look at the vents from a normal viewing position. Since
the metal is usually painted the soffit color, the vents become
essentially invisible. If the vents faced the other direction then
you'd see dark spots where the vents were.
If you can survive the cosmetics, theoretically, the openings should face
outwards, so as to provide the largest opening to the great outdoors. This
will minimize any convective heating or cooling of the air going through the
soffets because of the exposed exterior wall surfaces.
These surfaces can absorb a lot of heat from the sun or cold from ice/snow
below the soffet openings.
I admit, this may be a minor concern.
You also avoid radiant heat transfer through the soffet from the outside
walls.
Please feel free to comment.
Best regards,
Jack
Yeah, in theory facing out would be cooler, but I doubt that the
difference is that great, whereas the cosmetic difference is quite
significant.