I am looking for information on how to properly replace a bathroom fan which is not working well enough (probably insufficient CFM). The bathroom fan I need to replace is a 90mm fan rated for 65m3/h (about 38CFM) and is connected to a long PVC duct to the outside.
This answer nicely explains how to take into account the bathroom size and the duct length to estimate the needed CFM. I have not taken measures yet, but I think I will need a 110CFM fan (e.g. 560 cu.ft bathroom with 50feet equivalent duct length).
However, the long duct is also smaller than the fan: the fan is 90mm, while the duct I think it's 80mm (slightly more than 3 inches). The duct is connected to the fan with a duct increaser.
My question is: how to account for the smaller duct?
In this other question there is a table (IRC table M1506.2 Duct Length) that says 3 inches duct cannot be used for fans with 80 CFM or more. However, no answers were provided. Here instead, the answers say that it's ok to use a bigger duct for a smaller fan, however I am in the opposite situation (fan larger than the duct).
The new fan that I would like to purchase and install is even bigger (100mm, about 105CFM) but I do not want to waste money if the smaller duct will make the new fan useless.
Note: I cannot increase the duct size since it would require masonry work that I cannot do.