Most e/f's come equipped with backdraft dampers at their outlet...where the duct connects to the housing. I'm guessing yours got pulled. B/D dampers are required by code around here, but nothing prohibits the common termination.

 

Thanks -- I see the back draft dampers in the fan itself -- a friend (DIY'er but not a pro) suggests adding an extra back draft damper -- I'll think about it.

 

110CFM fan really needs a 4" duct to deliver it's rated performance. Shame to spend the $$$ on a nice fan just to choke it with a 3" duct. Be sure the roof cap is rated for the CFM of both fans combined. Read the installation instructions that came with your new fan, it may specify things we haven't mentioned here. Follow MFG instructions for optimum performance...

 

I'm disappointed with the 3" duct. I'm VERY new to DIY stuff so ... I didn't feel confident w/ changing the existing, rigid metal ducting (maybe I should consider a professional). The bathroom is 5'x8' w/ an 8' foot ceiling. Based on rough EDL calculation (but based on 4" duct), I'm hoping that the performance degradation isn't that bad. I'm waiting for a 3" semi-rigid duct to come in so ... once I get it, I'll let everyone know the results.

Here's my setup:

fan --> 4 to 3" Reducer --> (maybe a 3" back draft damper) --> ~ 36" straight run of 3" semi-rigid duct --> ~ 45 degree angle of same semi-rigid duct (9") --> 61" rigid, smooth metal pipe 3" diameter --> slight elbowing when attaching metal pipe to a common (common for both bathroom ducts), large-width pipe (common, large-width pipe is 12") --> Roof Cap (didn't measure - too high up)

 

By using 3" instead of 4" duct you turned you are cutting the fan's capacity by 1/2 what you calculated based on the 4" EDL. I'd be surprised if the fan actually delivers 50CFM, much less the 110CFM you paid for. Most of the Panasonic fans are rated for a 0.1" static pressure to get rated CFM. Is all the ductwork for the fan accessible? Did you DIY the fan install?

 

(Diagram attached)

The bathroom is small (needs < 50 CFMs actual). Fan install was DIY; in either case (DIY or prof), the goal was to work w/in a limited budget which didn't consider changing the existing duct (e.g. when considering prof install, the estimates were based on using existing duct work). The idea was to pick a fan w/ enough CFMs (based on a static pressure of 0.1) to provide the bathroom's needed CFMs after considering the reduction in the fan's CFM given the much higher static pressure of the existing duct work (w/ the 3" diameter ducts, the elbows, and the other items). If this reasoning isn't correct, please let me know - thanks.

The duct work is mostly accessible from an attic that has no flooring (walk on 24"-separated joists); 25" x 25" plywood is used for when I do work up there (sit on them). 'Mostly' means that if I were doing it, I wouldn't be able to access the top part of the rigid metal duct pipe, the common metal duct, and the roof cap. I am replacing the semi-rigid duct (about to order a 3" diameter semi-rigid duct pipe).

 

You BEST option is to have a pro come in and run a solid 4" pipe all the way out the roof/wall/soffit. This will get the rated output of the fan while minimizing noise (low noise is why you chose to upgrade the fan to begin with right?). If you can't run 4" all the way to accessibility issues, use 4" for as much of the ductwork as possible. The closer the 4x3 reducer is to the end of the duct the more total airflow/less noise you will get.