I have 2 in line fans that extract air from very small cellar studio along 3 metres of ducting to the outside of the property. They are inefficient despite being expensive! Is it that they are in the wrong position - they are both immediately behind the covered grills in the ceiling and therefore have to push the air all along th ducting to the outside. Are they better at pulling than pushing? Should they be nearer the outside wall?
The fans are better at pushing so should be located near the intake.
'They are inefficient despite being expensive!' If they are 4" fans and were less than £40 they are cheap and probably nasty.