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CFM planning for downdraft ductwork

Posted by bryguy11 (My Page) on
Thu, Dec 26, 13 at 10:57

I have a Dacor RV36 downdraft with 600CFM inline blower. The inline blower died recently (~10 years old) so I need to replace it.

I was never fully happy with the amount of air that the old blower pulled off our 36" gas cooktop. I see this as an opportunity to upgrade the blower to something more effective. (I know downdrafts are less effective than hoods. If I could redesign the kitchen I'd switch to a hood but that's not an option.)

Dacor makes a 600CFM inline and an 1100 CFM inline.

The 600CFM expects 8" round ducts. 8" round = 50 sq inches.
The 1100CFM expects 10" round ducts. 10" round = 78.5 sq inches.

The outlet from the downdraft unit is 3.25x10" = 32.5 sq inches.

So I'm wondering, given that the unit itself only outputs 32.5 sq inches, whether I'd see any benefit to the 1100 CFM unit over the 600CFM unit, given that the ductwork for both already exceeds the output size of the downdraft unit itself.

Additionally, the current duct run is somewhat suboptimal, with more bends than needed.

Given all this, I'm wondering if I should just stick with a 600CFM unit and make the ductwork more efficient. Or would improving ductwork efficiency, and upgrading to 1100 CFM be worthwhile?

Follow-Up Postings:

RE: CFM planning for downdraft ductwork

3.5x10" is a normal transition from the corresponding rectangular ducting, 3.5x14" would have been closer to maintaining the volumetric efficiency of the 600CFM blower, thats where I would aim, as well as eliminating those otherwise restrictive bends.

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