While all bends and transitions to other shapes cause small pressure drops that reduce the fan capability, if the rectangular duct has a proper transition to round at each interface, and is at least the area of the 10-inch duct (you can check this by calculation yourself), then the duct proper is not your problem. Wolf 1200 cfm fans can handle more pressure drop than these effects cause.

I would first question whether the damper that is either over the hood or at the exterior is opening. It may have a sheet metal screw blocking its function.

Second, the air flow rate you can move outside is never greater than the flow rate of the air you replace it with. Where is the hood getting its make-up air from? You need at least a window opening the size of the duct area when evacuating the cooking effluent.

kas

Thanks Kas. I will look into the damper this afternoon. I open my balcony door (adjacent to kitchen) or a window when I turn on the hood to create a draft.

A 10" duct is equal to 78.5" sq. in. ...so your "shallow but wide" duct must be at least the same... which could be 6"x 13", 5"x 16", 4"x20", or 3"x26" etc... check the dimensions of the duct to verify

You can add an inline blower past the hood to pull more air out of the duct work to make up for the bend that would have been ideal with the make up air damper

Ah! A blast from the past. One thing I would add based on more information over the years is that the rectangular duct has to be somewhat larger in square inches cross section than the circular duct to be equivalent. This is due to the rectangular duct having a larger amount of drag inducing duct 'skin' surface per unit area of its cross section than the circular duct.

One rule of thumb I have found is:

• equivalent circular duct diameter = 4 x (cross-sectional area) / (perimeter)

Alternatively, and better, is to use web sites that calculate actual pressure loss.

It would be helpful if @mawolfson returned to complete the story.