Heh! You came to the right place because searching for kitchen ventilation threads on this forum will fill your days with reading material. Along with various referenced sources, a gestalt of the supplied ventilation theory, facts, rules, and lore will be constructed that should be sufficient for you to optimally trade within the option space of performance (includes noise), cost, and aesthetics. (Generally only two out of three can be expected.)

Some maxims that are relevant are listed below.

• The purpose of the ventilation is direct capture and immediate containment.

• Capture requires a hood aperture that extends to the practical limits of the expanding cooking plumes from the burners; three inches on each unconstrained side of the hood is a minimum overlap; 10 degrees half angle from the locus of all hot pan bottoms is a better criterion.

• Containment requires (a) that all rising effluent that impacts a surface on its way to the filter reflect upwards, and (b) a flow rate at the baffles or mesh filters sufficient to keep all plume momentum when reflected at the filter still rising upward.

• Around 90 cfm/sq.ft. of capture aperture is normally sufficient; modestly lower can be argued for induction cooking. Other values can be derived from various rules of thumb.

• No blowers in any real configuration with less than perfect make-up air will flow at their rated flow rate. You should consider that for normal fan curves with nominal baffle, hood transition, duct, silencer, and MUA pressure losses, that the value obtained from multiplying 90 cfm/sq.ft. by the capture aperture in square feet should be increased by a factor of up to 1.5 to assure that the flow that desired is achieved.

• Duct diameter should support a duct air velocity of around 1000 ft/min (higher can be recommended if the ducting is normally warm).

• There is always MUA. If you seal up the house the hood flow rate will drop such that at the resulting house pressure the hood flow rate will be matched by whatever leakage is occurring in the walls and window and door edges.

• If you have unblown combustion appliances such as dryers, furnaces, and hot water heaters, then their air sources cannot be connected to the kitchen air if the house differential pressure is allowed to fall below -0.03 inches of water column (w.c.). This is to eliminate back-drafting of the combustion products which may include carbon monoxide.

• MUA can have levels of complexity that take too much space to address in this message.

To answer your question with an example: I have the largest Wolf Pro Island hood extending over a 36-inch induction cooktop and a Cooktek induction wok module. The capture aperture is about 10 square feet. The blower on the roof is rated at 1500 cfm at zero static pressure, and I estimate that with reasonable MUA an actual flow rate around 900 cfm is achieved. The wok cooking plume effluent is essentially fully captured. With the hood powered up to an appropriate flow rate, cooking odor is only noticeable when one's head is under the hood edge.

This system uses a 10-inch duct with a Fantech silencer.

At full power there is baffle hiss and a little fan unbalance rumble propagated to the hood area, but cooks can talk to each other at normal speaking levels, and talk across the peninsula to guests at normal speaking levels. Obtrusive fan blade turbulence induced noise and upper duct turbulence noise seem to be fully suppressed by the silencer.

kas

Re noise: do you think a straight 8 inch vertical duct from the hood, to a silencer in the attic space, to the Fantech external blower (on the roof directly over the hood) is advisable? It seems to me that any bends in the ductwork will lower the efficiency. But I have read that to lessen noise it might be advisable to have longer ductwork, with a 90 degree bend, then silencer, then another 90 degree bend out the roof, which would mean that the blower is much farther away from the hood. That configuration might use an in-line blower instead of an external one. I want it quiet, but I also want it efficient.