I think that there is a rule of that you need 1 CFM for every hundred BTU or about 750 CFM.

The hood is supposed to overlap the stove top by 3 inches on each side (I think that is a recommendation not a requirement but you'll need to check your local codes).

As I understand it you can have the blower (which is the noisy part) either local-meaning right in the hood over your head or you can have it further on down your duct-somewhere else so that it's much quieter. I have only ever considered local blowers so maybe someone else has experience of the remote blower.

Some local building codes require that anything over a certain CFM have a make up airflow and/or needs a permit.

To determine if you need make up air flow you need to do a whole house calculation...

Even though you have higher than normal ceilings, your hood should probably not be more than 36 inches above the stove top.

Since you have a higher than normal ceiling, you'll need to be careful when ordering as you may need an additional chimneypiece between the hood and the ceiling for that extra height.

My rule of thumb is that with a baffled hood, you should strive for an actual flow at blower full power of 90 cfm per square foot of hood aperture to keep the velocity up where the rising effluent strikes the baffles and can "reflect" out if not pulled through immediately. For a four foot by two foot hood aperture, that is 8 x 90 or 720 cfm. This is the air flow rate that should be possible given pressure losses getting air into the house, through the baffles, transitions, and ducts, and back out of the house. The zero static pressure rating of the blower will have to be higher than this number. A 1000 cfm to 1200 cfm blower rating will be needed, depending on myriad factors.

Duct diameter should be selected so that at the 720 cfm flow rate, the velocity in the duct is at least 1000 ft/min, but not more than 2000 ft/min. (The balance here is between higher grease condensation time and higher grease impingement velocity.)

You will want an exterior blower, in my opinion. If you have the room, an in-line silencer can be used, but will be fairly large in diameter (12 - 14 inches). (See Fantech's web site www.fantech.net for information.)

kas