I ran an experiment and the worst case pressure increase (closed room at the start of the ductwork from an HVAC unit with two inlet registers and no return) was 0.2 to 0.3 hPa. So I finally wonder no more about this 
I ran an experiment and the worst case pressure increase (closed room at the start of the ductwork from an HVAC unit with two inlet registers and no return) was 0.2 to 0.3 hPa. So I finally wonder no more about this