Hello,
I wanted to explain my contribution about the possible influence of supply air quality from contamination of the exhaust air pipes in more detail. The picture shows my ValloPlus 350 MV system.
As you can see, the exhaust and supply air streams are very likely not airtight separated from each other. The next picture shows the inner structure of the heat exchanger.
As you can see, the exhaust and supply air streams are only separated by thin fins. Of course, the question arises whether this construction will remain airtight over the years.
Another reason to assume that air from exhaust pipes can enter supply air pipes is the black ring around the plate valve.
The system has two filters (G4 and F7), which we change every 4 months, yet the black ring formed around the plate valve after about 1.5 years. Our heating installer said it happened because of the chimney outlets. He also said that he often observed this in houses with a fireplace. If this statement is correct, it means that the air from the chimney pipe is sucked into the supply air pipe. This is hot air, which theoretically should flow upwards during the colder season. The pipes on the roof can be seen in the picture.
The distance between the chimney and the supply air pipes is about 6 m. We use the fireplace no more than twice a month during the cold season. From this, I can conclude that air from the exhaust air duct (which is always in operation) occasionally enters the supply air duct.
Conclusion.
An airtight separation of supply and exhaust air streams can very likely not be guaranteed, i.e., the quality of the supply air may also depend on the condition of the exhaust air duct. Of course, everything strongly depends on the design of the system.
With this contribution, I certainly did not want to question the use of ventilation systems. We are very satisfied with the ventilation, and I think it is a great way to ensure healthy air in the house, provided that the system is professionally installed and maintained.
Best regards.