If a hood to duct transition is needed, then it has to be at the hood. A properly designed (tapered) transition should cause only minimal (compared to baffle, mesh, or MUA) pressure losses. To first order, I don't see an obvious reason for either type of transition to be noisier than the other type, and I don't expect either to be noisier than the baffle/mesh turbulence noise, fan blade tip turbulence noise, hood interior to transition turbulence noise, or main duct turbulence noise.

Hoods are often "designated" by the flow rate capability of their blower at zero static pressure (blower hanging in the air), a flow rate that one won't get even with the blower mounted in a hood that is hanging in the air. Without an analysis of the entire air path's pressure losses (and residential hood manufacturers generally won't tell you what the hood's pressure loss vs. flow characteristic is, along with the counterpart fan curve for the blower, then one can't estimate what flow will result from a given configuration without a lot of guessing.

Here is a reprise of my guess process:

• If possible, select a hood that overlaps your cooking zone by three or more inches all around (except at the back if against a wall).
• If possible, select a hood design with maximum capture aperture for its overall size; that is, with minimal internal area taken up by light panels and such.
• Calculate the area of the capture aperture on the cooktop side of the baffles or mesh filters (in square feet).
• Multiply by 90 CFM/square foot for desired actual full power flow rate into the hood.
• Multiply this result by 1.5 to get to the ballpark of a blower zero static pressure rated flow rate to use with the hood.
• For extra credit using the desired actual flow value, calculate the velocity of the air in the duct to see if it falls within the 1000 to 2000 ft/min range. (Divide cubic feet per minute by square feet of duct area.)

kas

Thanks, kas. I've done a little more research today. The Vent-a-Hood website states that one should never reduce the diameter of the duct below their recommendation but does not give a reason why. Best on the other hand has an expensive "iQ series" DC blower that supposedly adjusts the airflow to the ducting. I need a good enough solution not an optimal solution but don't want to make an expensive mistake.

So I think we are at either:
42" Ventahood 9" under cabinet 21" deep 600 CFM 8" diameter reduced to 7" duct

42" Best Centro Poco 10" under cabinet 22" deep 600 CFM 8" diameter reduced to 7" duct

So far I haven't found either of these on display in local showrooms (or anything like them).

A VaH website listing of one of their blower's performance parameters that another poster pointed me to sometime in the past showed that compared with the fan curves I had of some Broan blowers I had, the VaH blowers lost performance with pressure loss at a greater rate than the Broan's. (In other words, plotting VaH's data on a Broan fan curve chart would show a curve that fell below the Broan curve.) This may be due to the general behavior of squirrel cage fans vs. axial fans or centrifugal fans. Hence, shrinking the duct size would cause a VaH blower to lose actual flow rate faster than a counterpart Broan blower. (This may depend on the particular blowers in question, so both sets of data should be compared.)

What this means in real life is that the VaH blower should be sized larger if a higher pressure loss duct is to be used. Note that this is true for any blower vs. any pressure loss, including in particular the pressure loss of the MUA path. It is these losses that lead to my throwing in the factor of 1.5. If you can get any data or relevant imagery, try to estimate the aperture areas of these two contenders and perform the calculations I described above.

kas

Great, thank you. I'm going out to a few more local stores today to see if I can get a good visual on any of the models I'm interested in. I've found nice performance charts online for Prizer but there is only one dealer in my metro area of over 3M people and they only work with customers who are also buying cabinets.

Well I didn't find a Best to look at but I was able to cross Imperial off my list. The corners are sharp and there are a lot of exposed screws/seams.