Which chimney variant?

Then the chimney sweep and the chimney builder should probably have paid better attention in vocational school. What is nonsense about that? Warm air rises, leaves the chimney as flue gas, leaving a vacuum because fresh air is sucked in via the easiest route.

And that is exactly not supposed to work well - no idea. I didn't want something like that anyway, they just confirmed me.

The train cost me about 1500 euros.

Most manufacturers have a round insulated flue pipe. If there is air between the flue pipe and the casing stone, I have the option to drill into this for fresh air intake. I don't have to if I want to operate a room air-dependent fireplace, but I can if a room air-independent one is desired.

It also doesn't cost more, since the construction of the chimney is exactly like that.

If insulated, you can't draw air there... At least it then has no approval for that.

That is absolutely not true and is, in the words of your chimney sweep, "thin shit." The flue pipe is insulated, next to it the supply air flows past.

See for example tonatec plus

With my Schiedel, it is like this and not possible as you describe. Then it would also be dependent on room air.