Had a similar problem this week.
having been driving a ford for the past few years with a heated front screen and a very effective screen vent, i've found the screen demisting in the 147 somewhat lacking.
getting into a frosted car takes two to three minutes to get the mist shifted and it only really clears from the centre. seemingly the plenum behind the vent doesn't do a very good job at balancing the air flow to the full length of the louvre, even then, the flow is weak; once clear though, stays clear.......
until on a wet evening i decide to turn off the aircon off (and the re-circ) then the whole shooting match falls apart.......arrgh, oh well, as others have intimated, the aircon seemingly needs to be on at all times, hot, temperate or cold.
I used to be of the opinion that aircon cost fuel but having done a few warm, dry day runs, have found that there is seemingly so much torque from the jtd, that it makes no discernable difference.
I doubt there is much that can be done to improve the volume flow rate to the screen vent, even if you could affect a change to get a more even flow from it; Have fun trying though
Thanks for that
Can you feel the air coming out of the vents at the base of the windscreen? I find in mine there's no detectable airflow regardless of the fan speed. That's why I think I need to take the dashboard apart.