I will soon be looking to replace the extractor fan in bathroom, the current one goes in to the losft and exits who knows where (if at all) everything is buried under a foot of insulation, although the insulation installer did state a pipe went to the soffit but externally I cant see an exit. No matter as I would prefer to use this if possible (image) which is a ridge vent, left over from I believe the old heating system. Distance from vent to fan approx 15 feet and planning to use an impellier fan. The question is the metal vent pipe I believe is 4.5 inches diameter and at this stage not sure if I want a 4" or maybe 6" fan, so should I be able to get either a reducer 4.5 to 4 or a 4.5 to 6" to be able to connect the pipework?
Any other issues I should be aware of (I know about a condensation trap)