Bathroom exhaust fan

[Boxxed]

My bathroom is about 50sq ft. with 4" rigid running horizontal about 25' with a 120CFM exhaust installed. It did not seem to clear the room that great. So, I put in a 300CFM fan thinking it would be better but notice I am getting blow back from that and it seems like it made it worse. (I was getting a slight amount of blow back from the 120CFM fan as well)
It seems like the problem is the 4" and the distance of it and/or there is not a 1/2" gap under the bath door. The ductwork is in the drywalled ceiling of the basement.
Considering 1CFM per square foot of room space puts me at a 50CFM fan. I was thinking more is better. ( if I understood that correctly)
Would I be better off to put in a 70CFM exhaust fan to match the current ductwork or install 6" ductwork? And cut some off the bottom of the door? I can test that if I keep the door open. Replacing the ductwork to 6" requires I cut the drywall or cut the subfloor above to replace.

Thanks..

 

[Fitter30]

Take a look at this site good explanation on sizing.

Bathroom Fan Duct Length and Dimension Guide (Tables & Charts) - The Tibble

Functionally, the purpose of a bathroom fan duct is to conduct exhausted air from the bathroom to the outside. Practically, the duct has to be sized so as to create …

hvac-buzz.com

 

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[Boxxed]

Take a look at this site good explanation on sizing.

Bathroom Fan Duct Length and Dimension Guide (Tables & Charts) - The Tibble

Functionally, the purpose of a bathroom fan duct is to conduct exhausted air from the bathroom to the outside. Practically, the duct has to be sized so as to create …

hvac-buzz.com

Thank you.. I think this is what I have going on "Remember that increasing the fan capacity is not necessarily going to allow you to run a longer duct system unless you increase the duct diameter as well. This is because a more powerful fan without a larger duct will result in more friction and greater pressure loss. It will also be frustratingly loud."

I will probably cut the subfloor out to replace the 4" with 6" instead of cutting out the drywall. The SFloor is easier to get to at the moment.
According to the link you sent me it seems like a 50CFM fan would work with the 4" that I have. My only concern is that the 4" is about 25' to exterior vent..

I see what the guide says about a 300CFM fan and 6" ductwork, fittings will exceed rated pressure drop. To be clear, I will not get a better draw from the fan that uses 300CFM through 6" then if I use 80CFM through 4" ductwork?

Also, according to the link It it was also originally set up wrong. The Rigid duct length sizing table explains that 4" duct with 0 elbows can go 31' with 80CFM fan.
I was running 120CFM fan and it was not good. I could have left the fan and replaced the 4" to 6" according this and would have been fine.
Or Replace the fan with a 80CFM and be done with it.....
Does that sound right?

Thanks alot.

 

[Fitter30]

Sounds good to me. Look at the exhaust vent hood pick one that looks least restricted. Doesn't matter talking air or water every fitting and pipe has a pressure drop when flow is excessive it gets noisy and under performs. Glad you understood the article. Angle grinder with thin wheel will cut the pipe easley. Hand crimper for making the end smaller and metal tape to seal joints.