Due to recent circumstances, I finally opened up my system after 7 years of continuous operation to change the filters again and clean the heat exchanger.
This is what it looks like:
At the bottom right is the fresh air chamber, which means unfiltered air from outside comes in there.
You can see that a layer of dust is present both before and after the filter.
At the top left, however, after the heat exchanger (fresh air into the rooms), everything is relatively clean, almost like the first day. I can also tell by the supply air vents in the rooms. They are still clean.
At the top right (exhaust air from the rooms), there is also a slight dust layer. But significantly less than at the bottom right.
After I rinsed off the heat exchanger, some dirt of course came out.
Take a look at the setup of my system. All four air chambers are hermetically sealed from each other. Practically nothing can flow over.