Can you run two, 8 ducts? Some hoods, such as the Prestige 1200CFM pack, have two 6 outlets that are generally transitioned into a single 10 duct. Depending on the obstruction you need to get around, it might be possible to transition to two, 7 or 8 ducts "immediately" and then put a two-into-one transition past it, or have two vent caps.

There are huge differences between a 10 duct and an 8 duct -- just looking at cross-sectional area the 8 duct is 64% of the 10 duct. It is even a little worse than that because of how air moves through a duct.

(sorry no inch marks, it wouldn't post otherwise)

The available space in the ceiling is 9" X 11"

It WOULD be 11" x 11" but there is a copper pipe crossing the space about 2" from the ceiling (leaving 9" above it)

Also, when we got ready to exit the exit the exterior, we found a header. Structural engineer recommended putting a 2x12 between the joists to secure them against lateral movement. Therefore, only 9" height at exit point as well.

Modern Aire says their 1200 CFM is supposed to go with a 10" but would be OK with a 9" duct. Price, shipped, is the same as the VAH. That is the leading option.

We recently installed a Zephyr AK8300ASX hood with 1000 CFM internal blower and it uses an 8" duct. It works great over our 42" Viking range. It also comes with this cool little remote control with a magnet on the back so we put on our fridge and can control the hood without having to reach up and change the speeds.

Stephanie: 12'' duct is very unusual in residential units. All units I've seen have spec'd 10'' duct as a rule.

One option to discuss with your HVAC contractor would be to use rectangular duct for the constricted area. It's a bit more constrictive than round duct, but, for example, a 18'' x 4'' rectangular duct will squeeze through small spaces, and then you can run 10'' duct on either side of that. While it will act as a bottleneck, its effects should be modest, and certainly superior to using 8'' or 9'' duct throughout.

In either case, you are best off with a better quality internal blower. The one that MA uses is designed to compensate for back pressure of long duct runs and will maintain flow rates even through rather high resistance. May be a bit noisier than 10'' duct but remember that with the MA you'll have an infinitely variable speed control and you'll find that you run at far below full speed most of the time anyway.

I have little doubt the MA with 1200 cfm internal blower will outperform the VAH--particularly if they are quoting their mysterious "equivalent cfm" which means the actual cfm rating would be only 800 cfm to start.

PS: sfjeff: two single quotes ' ' typed in succcession will be "seen" by GW as double quotes, for the "inch" sign.

Consider using an externally mounted blower system as distinct from internal - generally, dependant on the lenght and number of bends in a duct run, sucking is more efficient than blowing - if the external motor has to work that bit harder due to a smaller than ideal duct size it will be outside and the mechanical noise wont be as bad as inside. Downside is external mounts are more money. Also ensure that you use baffle style filters to further assist the smooth air flow and up in the 1000 cfm range less airflow noise through baffles than mesh filters.
Other option, as suggested by sfjeff, is to run two ducts - Modern Aire can custom the exhaust on the hood to feed two ducts with custom collars.

We should be careful to distinguish noise from working hard. A blocked blower does no work, other than spinning itself and generating some local turbulence, so the motor is not working very hard. It might be noisy, however, due to the turbulence around the blades.

While a large flow rate is desirable, particularly for the larger hood sizes, the duct versus fan sizing should be matched so that the velocity of the air in the duct (at maximum flow) is in the 1000 ft/min to 2000 ft/min range.

A 1200 cfm (zero static pressure rating) blower using a 10 inch duct, with hood, exit transition, and other stuff causing pressure drops is unlikely to actually move more than about 800 cfm unless the ducting length is very short.

For example, 800 cfm / 1600 fpm = 0.5 square feet or 72 sq inches. This is the area of a 9.6 inch duct. A 9-inch duct would be a tad on the low side for the selected velocity, but a 9 x 11 duct should be fine. Ignoring its slightly greater surface area and hence loss, the rectangular duct would provide more area than a 10-inch round duct (78.5 sq in).

kas

My installation has a lot of restrictions.

I can't do an external blower. My house is quite contemporary. The duct emerges on an exterior wall with over 40 casement windows. Even the smaller external units are too obtrusive. The duct runs between 2 floors with no ceiling access from above. No attic or roof access.

Thanks for the techie info. I appreciate it.