Ducting size for inline extractor fan

Hi Guys, new to this site, please go easy on me :-)

Im planning to upgrading my inline extractor fan in my bathroom/shower room. Current setup is manrose shower light 125mm 130m3/h, 2.5 meters flex ducting to vented tile. 130m3/h is the size calculated dead on for my bathroom. Thought I would go abit bigger i have purchased an airflow aventa (2 speed options) 145/187m3/h. This fan is 4". Im keeping intake and exhaust at 5".

My question is should I run 5" ducting and reduce each side of new 4" fan or run 4" ducting with reducers at intake and exhaust. Not really up on the science, air flow verses air velocity...… resistance ……

Is there much in it?


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