ᐅ Wall box for kitchen exhaust vent

We have a rectangular wall box with a 125mm (5 inch) duct passing through it, as well as a second flap that simply allows air to flow either from outside to inside or vice versa. I’m wondering if this second flap is necessary. On the same floor (but in a different room), we have a fireplace and did not connect any tilt-window switches to the ventilation duct there. Is that why, or are there other reasons? I would like to switch from 125mm to 150mm (6 inch) because my kitchen specialist told me that with 125mm, the airflow maxes out at 500 m³/h (cubic meters per hour), and even if I use a hood with higher capacity, the air gets stagnant and nothing really works. What do you think about this statement?

The statement is accurate—I would agree with that.

What qualifies you to make this statement?

@Soroka: I believe that was an expression of opinion. That's how forums work.

Now, opinions should be qualified. I cannot see any qualification in the seven one-sentence posts. The original poster (OP) can probably do without such "opinions." After all, I also hold back if I have nothing meaningful to contribute...

125mm (5 inches) and 500m³/h (300 CFM) is really borderline.
The hood will work, but it will be somewhat noisier than with 150mm (6 inches).

We had a customer where, out of necessity, a 400m³/h (240 CFM) hood was connected to a 100mm (4 inches) duct.
At level 3, the hood was unusable. The backsplash behind the cooktop was soaking wet after just a few minutes. On level 2 it was better, but still not satisfactory.

If it is not too much trouble, you should upgrade to 150mm (6 inches).

Good luck