If I reduce my bath fan duct from 6" down to 4" for a length of 5" at the exhaust point, will it have any meaningful effect on back pressure or CFM?
Is it even worth it to run 6" duct for the majority of the run if it will reduce down to 4" at the end?
I have a Panasonic exhaust fan that will compensate and run faster to maintain CFM if there is back-pressure on the duct. I would like to have the fan run as low as practical to save power and reduce noise. I cannot replace the 4" outlet that the fan uses to escape the attic without great expense, so I am trying to figure out if it really matters.
The duct run from the fan is insulated flex duct. The 4" duct to go out the louver is rigid.
References I have already found on the topic are not totally clear on this point.