Wall or roof penetrations (external/supply air): how did you install them?

You simply need two of those

That is wrong, you only need one. The Vallox weather protection grille is for intake and exhaust air, so you could install it on the south side at your place and wouldn’t need a grille on the north side at all.

It has been working flawlessly for us so far.

What, you have both in one box?

Yes exactly, the geometry of the nozzles is designed to prevent the mixing of the airflows. Vallox also has a graphic of the flows, you should google it.

Yes, I know such things - but I simply don't believe in them. Especially with poor air quality in the house from certain corners...

Everyone talks about the ventilation short-circuit, but is it supposed to work there?

As I said, we have no problems and fresh air in the house at all times. Especially with weak air currents, you can't get far with your intuition either, for example, I would have initially thought that the supply air vents above the room doors don't make sense...

That also makes no sense from a ventilation perspective because these are weak air currents that cannot ventilate the room...

Especially when you have a system for the first time, you think that everything is great – it is definitely better than without. In our rented apartment, there were also great things like supply air above the door, wrong plate valves, etc.