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Hood calls for 7' duct, can it connect to 8'?

Posted by marti8a (My Page) on
Sat, Jul 21, 12 at 19:37

Long story short, the hood liner I want is a Modern Aire PSL 234 that calls for an 8" duct. But for the time being, I am going with a Broan that calls for a 7" duct.

Can I put the 8" duct in and use an adaptor, or put a 7" inside the 8"? Or is there an alternative?

Follow-Up Postings:

RE: Hood calls for 7' duct, can it connect to 8'?

If you are pretty sure you will be upgrading later, by all means put in the 8 inch diameter duct and use 7-inch to 8-inch adapter to connect the Broan hood for the time being. No harm in going from the hood into a larger diameter duct. Probably will run a bit quieter, too.

RE: Hood calls for 7' duct, can it connect to 8'?

Good. I didn't think it would be a problem but wanted to make sure.

RE: Hood calls for 7' duct, can it connect to 8'?

For residential applications where the air flow rate varies, and the ducting can vary in temperature, the only important factor is sufficient diameter to minimize pressure loss through the duct. Seven inches transitioning to eight inches is fine.

If one wokked or grilled a lot and ran the fan on full power (and one knew from analysis what the flow rate really would be as a function of duct diameter), then for cold climates with cold ducts one wants the flow velocity in the duct to be in the 1000 - 2000 fpm range (smaller duct), while for warm climates, 1000 fpm or lower is better (larger duct).

The reason is that higher speeds cause increased grease collection in warm ducts due to turbulent impingement, but slower speeds cause increased grease collection in cold ducts from condensation. YMMV

In either case, given hood baffles or mesh filters, it would take a long time for enough grease to build up in the ducts to be a concern for typical residence cooking volumes.


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