Wall box for extractor hood

We have a rectangular wall box through which a 125 mm pipe passes, and also a second flap that simply allows air to flow from outside to inside or vice versa. I wonder if this second flap is necessary. On the same floor (but in a different room), we have a chimney and no tilt-window switches are connected to the smoke extraction. Is that the reason or are there other causes? I would like to switch from 125 mm to 150 mm because my kitchen master told me that at 125 mm, 500 m3/h per hour is the limit, and even if I use a hood with higher capacity, the air builds up and nothing works anymore. What do you think of this statement?

The statement is correct - I would sign off on that.

What qualifies you to make this statement?

: 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 7 posts of one sentence each. The OP can probably do well without such "opinions." After all, I also hold back when I have nothing meaningful to contribute...

125mm and 500m³/h is really borderline. The hood will work, but will make a bit more noise than with 150mm.

We have, out of necessity, connected a 400m³ hood to a 100mm duct for a customer. The hood was unusable at level 3. The tile backsplash behind the stove was soaking wet after a few minutes. At level 2 it was better, but not satisfactory.

If it is not too much effort, you should upgrade to 150.

Good luck