If you want to go up the interior wall, I suggest that you build the wall with 2 x 6s to provide 5.5 inches of internal gap. Then a 4 or 5 inch by 12 or 14 inch duct will fit. This will cost two inches of room on one side or the other. Eight inch circular duct has an area of about 50 square inches. 4 x 14 is a bit more (56), but square duct has more frictional pressure loss than round duct. If I were doing it, I would use fire-code sheet rock on those walls, and insulate the duct with mineral wool. This would minimize possible rattles from the blower, which I would put on the roof if possible, and provide some slight additional protection in case of a grease fire. Use only galvanized steel duct (stainless steel would also be OK :). kas

"Kobe is the one firm that talks about quietness." David - Where did you ever get that idea?!?! For one, Ventahood quite famously advertises their being the quietest ventilation available, and taken at face value their sone ratings at high speed are lower than Kobe's. Of course for some "strange" reason neither company chooses to be rated by the industry standard, HVI, so that we can objectively compare them against the competition. Broan is another company that talks about quietness, and they've got the HVI certification to back it up. At low speed (150 CFM) their Elite series measured in at 1.5 sones, which seems very respectable. The 13.0 rating at 600 CFM seems scary compared to Ventahood and Kobe's claims, but then who's to know if those brands were tested in a similar manner. As I see it, for many of the popular brands we're left with little basis for comparison other than first-hand observation and anecdotal comparisons found on forums such as this. "Once the other manufacturers pay attention to noise as a single most important criterion, I will be glad to add their names to my list of recommendations." Of course this is a matter of opinion, but in my book a quiet range hood is worthless if it doesn't get the job done. So I'd put performance first.

what size is the duct in your current hood vs. the new duct diameter you will need? not sure if it's possible but if the new diameter is 3-6" larger then you should be ok, no? ie, the hood we installed needed a 10" diameter which i was told was larger than normal (at least by the duct guys we purchased from). we weren't replacing an existing, but if for example our existing was 6" then we may have been ok in your scenario because the larger diameter would cover the new opening.

I am the resident expert only because real HVAC systems engineers don't frequent this forum, as far as I know. Noise introduced into the kitchen will be significantly reduced if an external blower with silencer can be fitted. A lot of the high frequency blade tip turbulence caused noise will be removed. This is also true of an in-line (duct mounted) blower. The issue of the neighbor is difficult to assess. What is the configuration of his or her windows relative to your potential external blower (1500 CFM I assume you meant). Your own window sound transmission is also an unknown. Generally, the external noise could be abated by (a) using a silencer on each side of a duct mounted in-line blower, or (b), using a deliberately under-driven* "up-blast" commercial blower (mounted as a side blast). (I think opaone is using an under-driven blower for his new system.) It is also possible to use a chase to move the blower up to roof level. I have a 1500 CFM nominal Wolf (Broan) blower on my roof. Its sound level is detectable but not obtrusive from the ground, but it might be if in a wall location where the sound is "trapped" between two houses. Possibly a more expensive but similar in design Abbaka downslope unit would be marginally quieter. ------ * commercial blowers of this type can use a pulley system between motor and fan that allows for different ratios. By choosing a ratio that operates the fan blades below their rated speed when the motor is receiving maximum voltage, significant turbulence noise can be avoided. The physical cost is a larger unit than would otherwise be required for the desired flow rate. Residential blower fans are directly coupled to their motors.