I don't know how the 190 works, but there is a description of when air is supposed to flow from which vents at the thread
Blower motor replacement procedure for 300E
This implies that there should occasionally be times when there is reduced or no airflow from the center, at least on the 300Es. Good luck.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E