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Need Vent Duct Advice....10 inch connected to 8 inch?

13 years ago

We are currently under construction and the HVAC contractor placed an 8 inch duct in the kitchen to vent the hood to the outside. The runs is approx 25 feet with only one 45 degree turn.

Here is my dilemma:

Due to very tough building inspectors in our town, the hood CFM rating has to be at least what is recommended in the rangetop owners manual. We are installing a 36 inch Capital Precision rangetop, so 1200 CFM is what is recommended. The Vent-A-Hood we have chosen (and are getting a very good price on) requires a 10 inch duct. It is a 900CFM model that is equivalent to 1350 CFM according to Vent-A-Hood's literature.

The HVAC contractor is saying we can run 10 inch off the hood and then "choke it back" to the 8 inch for the majority of the run. He said this may increase noise levels but should work fine.

Any tips are input on this?

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