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During construction of my completed-in-1997 home, I was able to upgrade everything as I wanted to except for the bath fans. Just didn’t think about them given the millions of other decisions to make. (This is the 3rd house I’ve built, btw)
My Broan 688 bath fans are rated at 2.0 sones, 50cfm and use 3″ ducting. I want to replace each of the fans with Broan S50U, 50cfm, 0.5 sone units, but the S50U is spec’ed for 4″ ducting.
I really don’t want to have to replace the roof cap in order to move from 3″ to 4″ ducting. If both fans are rated at 50cfm, why would the duct size increase? Is the larger duct size needed to reduct wind turbulence-related noise up the duct?
If I used a reducer (4->3) in the attic, as far from the bath as possible, would it make a difference in the ducting ability?
Replies
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The quieter fan may be quieter, in part, because it is doing less work on the air - creating less pressure and consuming fewer watts. Hence it needs less resistance in the duct work to move 50 cfm. A look at both nameplate amperages might confirm this.
I expect it would still work fine in 3" ductwork but might move 35-40 cfm instead of 50. The further you can run the 4", the better. The smoother the transtion, the better - conically-shaped, not right angles. Smooth-wall rigid metal is MUCH better than flexible ductwork due the surface roughness. PVC ductwork is even better than metal because it lacks seams and the joints are smoother. Few bends are better - two 45's to offset are better than two 90's.
Bottom line: Go ahead and install the new fan. Upgrade the portions of the ductwork that are easy. Use the smoothest pipe and straightest run you can. Then you'll get 50 cfm or close to it.-David
*Paul,The Broan Lo-sone fan is quieter because it comes with a little heavier fan construction and an insulated cabinet. Air @ 50 cfm in a 3" duct is not the source of the noise (at 1000 fpm, friction noise is negligle). You can leave the 3" duct work if: it is not flexible; the run is relatively short and straight. If it were my house, I would replace the 3" with 4" and leave the roof jack alone.If you really want your exhaust to be quite, consider an in-line fan to get the motor and blower (the source of the noise) away from earshot. Fantech and Broan make reasonably priced versions.
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During construction of my completed-in-1997 home, I was able to upgrade everything as I wanted to except for the bath fans. Just didn't think about them given the millions of other decisions to make. (This is the 3rd house I've built, btw)
My Broan 688 bath fans are rated at 2.0 sones, 50cfm and use 3" ducting. I want to replace each of the fans with Broan S50U, 50cfm, 0.5 sone units, but the S50U is spec'ed for 4" ducting.
I really don't want to have to replace the roof cap in order to move from 3" to 4" ducting. If both fans are rated at 50cfm, why would the duct size increase? Is the larger duct size needed to reduct wind turbulence-related noise up the duct?
If I used a reducer (4->3) in the attic, as far from the bath as possible, would it make a difference in the ducting ability?