"Have you thought of changing the wall studs to 2x6" rather than 2x4"?" I don't wish to destroy the wall on the far side. I have a large map wallpapered to the other side. I could build out with a false wall for some additional distance, but I don't see that as necessary since I can do at least 3-1/2"x14" (almost equivalent to a 8" round duct). Plus, I found a standard "Straight Register Boot" to transition to 8" diameter. I believe 3-1/2x14" equates to about .05 friction loss for five feet). I just don't know what 2200-2500 FPM will equate to in sound level at the hood. I foresee cutting a 4x15 opening (or bigger), at the back of the hood. I am re-reading rjpjnk's post and I remind you I only have five straight feet till it feeds straight into the fan venturi. I found this is an engineering handbook: Duct velocity should be between 1500 and 4000 FPM  Hood velocities (not less than 50 FPM over face area between hood and cooking surface)  Wall Type - 80 CFM/ft2  Island Type - 125 CFM/ft2  Extend hood beyond cook surface 0.4 x distance between hood and cooking surface Filters  Select ï¬lter velocity between 100 - 400 FPM  Determine number of ï¬lters required from a manufacturerÂs data (usually 2 CFM exhaust for each sq. in. of ï¬lter area maximum)  Install filters at 45 - 60°to horizontal, never horizontal Using this as a guide I think an average 400 CFM for general-purpose venting @ 1500 FPM means that I am well covered with a variable speed 1000 CFM external fan and 8" duct area. I'm guessing that at 1500 FPM it will be pretty quiet. The old Vent-a-Hood units are supposed to deliver 300 CFM each and I usually only use one. They are super noisy at near my ear level and 18 inches away. The air passes through a 3-1/2" x 4-1/2" rectangular opening! That's small and seems to equate to almost 3000 FPM! I guess it's no wonder they're so noisy... Am I missing anything? Bad calculations or asumptions?

"Finished" my installation today. I opened the wall and added in framing so that the mounting strip is held by 4 screws (the hood is held onto this strip by 2 screws). The roof vent (VentaHood, stainless steel I think) was rather easily installed as the roof has battens and spacing was wide enough for the hood to slip in between 2 battens and just replaced a tile. Straight up 2' from hood to ~65 degree elbow, 5' 10" to 2nd elbow (~85 degree) and straight out. All joints and seams metal taped and the ducting wrapped in insulation. Total vertical rise of ~6'. The roof vent has a flap held down by a magnet. Was worried after reading threads here that the distance and curves were too long and the air velocity and force would be too weak. 1 fan at half speed lifted up the flap half way and full speed all the way. Totally pegged with 2 fans running at full speed. The vent is a PITA to use as the flange is too short (just 1" so screwing in the elbow wasn't the easiest). Tomorrow, I will try making Cajun blackened chicken (Paul Prudhomme recipe) to see how well it works. One other note, the vent doesn't come with a pest screen. VentaHood advises against using one but birds and wasps are nesting pests where I am. I fabricated and will try a removable one and see how that works. Complication was that one of the lower screws for the warming shelf didn't hold (anchor failed) and the stainless steel flexes too much so I will have to open up the wall behind to add in a piece of framing. I will figure out a design for a duct cover (most likely will suspend it from the ceiling) and get that fabricated.

On the second half of your question, almost everyone here will tell you NO and I would agree. Most don’t even vent, they just recirculate. On the the first part, there are experts here better qualified than me, but there is a significant difference between 200 cfm and 400 cfm. Unless you have a non-standard duct, I don’t see a problem going higher. Hopefully others will chime in. Good luck!

Please note that you should ideally have a 36" hood over a 30" range, in particular if you have a gas range. Around here, most of the contractors already know that and this seems the standard configuration in new construction. In other parts of the country, contractors frequently don't inform themselves as much about what it takes to make a kitchen really functional vs. just making it look pretty. So, it is often on the home owner to point out these technical details. As you mentioned that you want a hood with a deep capture area, I'd like to give a recommendation for ModernAire. We have one of their hoods and are extremely happy with it. Due to various constraints with a deck right above the kitchen, we couldn't get a remote blower nor an inline filter. So, things are a little noisier than we'd ideally have liked it. But since ModernAire builds all their hoods to order, we got exactly what we wanted. Our hood it mounted above head height and is extra deep to easily capture the front burners, even when you consider the expanding plume.