I'll leave to others to report on vent hoods that I've never owned, but I will leave a recommendation: Use a blower that can achieve an adequate flow rate at the larger pressure drop that a constricted duct system will cause. Ensure that at full power at that pressure drop, the air velocity in the ducting is between 1000 and 2000 ft/min (for grease deposition control).

Velocity equals flow rate (cfm) divided by duct sectional area in square feet. The flow rate you determine to use is not the reported open air zero static pressure flow rate of the blower, but the cfm found on the blower's fan curve for the pressure drop you calculate for the hood and ducting. Because this pressure drop, a.k.a. loss, depends on flow rate, it is represented as a line on the fan curve graph starting at the origin that intersects the blower's fan curve. Broan has fan curves one can download from their website that can be used to see how this all works.

In short, you may need a blower that is rated significantly higher than the flow you intend to achieve.

kas

kas - I had to read that a few times, but I think I get it. I didn't see the fan curves on the Broan site. Do you have a link? I think you are saying I should beef up the motor to force more air through smaller area. Won't that cause a noise issue?

I was told by an appliance guy, that the rule of thumb is 100 CFM per 10000 BTU. I was just going with that and looking for a hood that is at least 600 CFM and engineered for a 6" vent pipe. It seems that many of the hoods require 7-10" for the pipe. It's been a bit difficult. I think even the Bosch hood that "matches" the cooktop requires 7". It was suggested to just go with that and add a fitting to reduce the pipe size, but that is reducing the area by over 25%.

Also, there are a lot of brands for hoods. I have to pull specs to find out if it will work in my set up. Is there a better way to narrow this down. Is it best to go with a recognizable brand or can I find a decent "generic" hood?

Kobe has 600 CFM hoods that work with a 6" duct. I know they have 30" hoods - don't know if they have 36" hoods offhand.

I have this one: http://www.koberangehoods.com/product/RA0-1.html (36" version) and the installation guidelines asked for 6" duct work.

OK. Apparently I've been looking in the wrong spot. I went to a Kobe site and there were 30 hoods with 6" vents. It didn't show any dealers in GA, so I would have to order on-line.

Gigelus- I assume you like your hood. That link didn't work to show your model, but I see a variety of nice looking units. Are there things you love or hate?

I've been living with a very noisy Jenn-Air downdraft. We are finally remodeling and the space will be open, so that is why I'm sensitive to how loud the unit will be. I know some units have external blowers, but that is not an option. What other features should I look for? Is there something that I should absolutely avoid?

On the theoretical side, yes, the more velocity in the duct the higher the turbulence noise. But I was only urging standard practice in duct velocity, not overkill. Even 2000 ft/min in a six-inch duct is only 392 cfm, which is likely about what you would get from a 600 cfm rated powered hood with additional duct losses and imperfect MUA.

There are several rules of thumb for required flow. Some are derived from practice but disassociated from the physics. I prefer to use effluent vapor velocity as the starting point. BTU/hr is an indirect basis, and derives from commercial frying surfaces. Anyway, a 600 cfm rated hood should be adequate for your need.

We could address overlap of the cooking area, but that is a different topic.

kas

There are no Kobe dealers anywhere near me, so ordered my Kobe hood online from ventingdirect.com. They were great to deal with. When the hood arrived, before signing for it I carefully inspected the box, then the delivery guy helped me open it and pull it out far enough to see that there were no obvious dents (which was nice of him). It was fine. The only thing I would point out is that ventingdirect two years ago used to say "10% discount until such-and-such a date" then when that date passed, the discount would get renewed to the next date. So don't feel rushed to order before you want to. There are other online options, too, and you may get a better price on a different site.