Hello I have a newly created bathroom in the basement and it’s time to finish the ducting of the bathroom fan. I have a ceiling vent fan with a 4 inch outflow duct. I need to run that duct about 25 feet through a joist channel until it will penetrate out the back of the house.

Due to the long run of ducting, and the low CFM of the ceiling vent fan, I was thinking about adding a booster fan (inline duct fan). This would be located in the joist channel, but in an unfinished mechanical room so access to it will always be available.

I guess my question is about duct sizes. Coming out of the bathroom ceiling vent fan is currently a short length of 4 inch duct, but the rest of the run I could step it up to 6 inches... is that good to do? Would that increase overall ventilation and airflow from the bathroom? I noticed that the 6 inch fans are only a few dollars more expensive than the 4 inch fans, so I was wondering if it would make sense to do that and boost up to 6 inches after the existing 4 inch section.

thank you