I have a Brink Renovent heat recovery ventilation system and I've opted to put an F7 pollen filter on it as well as the standard G4 filters. (The G4 filters are a thin foam-like material, whereas the F7 are like paper in a concertina form).
There are 2 G4 filters, one on the intake from the outside, and one on the outlet where the heated supply air comes out of the machine. I get the impression that the F7 is normally fitted to the intake (i.e. filtering before it gets to the machine), but is there any reason why this is so? Could it be fitted to the outlet on the other side instead? I ask because the G4 filters are washable and cheap and the F7 filters are *very* expensive and not washable. So I prefer to let the washable, cheap filters pick up as much dirt as possible so that I don't have to change the F7 so often.