Looks like I have to write a question before I can read any reply. What a stupid system.

I just try to find out where the grey/black "film" on near the ceiling at my walls is coming from. We regularly open windows to let fresh air in and I recently read that this has to do with evaporation of paint and floor board.

Can anyone help? Thanks

It could very well be a problem with your furnace heat exchanger if so equipped. It would be best to have it checked by a on site professional.