ding-dong, kaseki calling...

It is useful to look at this challenge from two points of view: first the behavior of the pans and cooktop, second the desired behavior of the hood.

When the pans are cooking hot substances, steam and grease vapor are generated. These plume constituents along with odor, being hot, rise upward. The rate of rise can reach 1.2 m/s, although most cooking plumes will be somewhat lower in velocity. This effluent plume not only rises, but also expands as it rises. A useful value for the strongest part of the plume is a 10-degree expansion angle. This is measured at all parts of the heated pan surface and is the angle relative to vertical. So basically this model looks like a truncated cone with the truncated part at the pan and the wider cone base at the hood.

The actual plume is more complex and has wider and slower rising effluent at greater angles. In most close hood conditions the plume fringes are pulled into the hood due to being entrained in the weak hood airflow near the cooktop. This likely won't be a successful gambit when the hood entry aperture is six feet away. More likely, such effluent will be removed slowly as the room air is replaced, but not necessarily before grease condensation onto walls and ceiling has occurred.

The function of a kitchen ventilation hood is capture and containment. Capture means keeping the plume from escaping the hood; containment means moving the plume constituents beyond the hood filter (mesh or baffles). Fire stop and grease collection are additional functions.

To accomplish this, normal efficient hoods (i.e., commercial hoods) generally have a capture volume, a filter set above this volume, and sufficient airflow that whatever plume enters the hood entry aperture into the capture volume leaves the volume only one way, through the baffles and thence to the outdoors. Residential hoods, to the extent that they more or less conform to this configuration, more or less successfully perform the hood function.

So we can see two explicit requirements: An entry aperture that extends over the cooking zone sufficiently far to meet the 10-degree parameter, and a flow rate that makes the baffles effective against the fastest rising plumes. In the latter case, I recommend 90 ft/min, which is equivalent to 90 CFM per square foot of entry aperture. Note that entry aperture area is normally larger than the baffle area.

Additional considerations for ceiling mounting.

• There has to be a filter system. A mere register or diffuser will not cut it.
• A lower air velocity may be acceptable on the grounds that the plume will have cooled after rising 5 or 6 feet. Maybe 60 ft/min would work if there are no drafts and there is a hood entry aperture capture volume (in the ceiling perhaps).
• Any drafts will deflect the rising plumes requiring a larger entry aperture to ensure capture, and hence a larger air flow rate and MUA flow rate.
• Filter cleaning can be a chore, and there is risk whenever one is working above a Ceram cooktop or stone counters.

Commercial hoods typically have entry apertures at seven feet above the floor. Let's see what might be required for 9 feet. Assume that pans can extend to the edges of the cooktop, and that the cooktop is 2 ft by 3 ft. Six feet of height times the tangent of 10 degrees is about a foot; hence, the hood entry aperture should overlap the cooktop by 1 ft all around. In other words, the entry aperture should be 4 ft by 5 ft. At 60 ft/min and 20 sq ft, one would need 1200 actual CFM. I would want an 1800 CFM rated blower for this purpose, and active MUA to keep the house pressure near ambient at that flow rate.

Other schemes are possible, including perforated ceilings and ceilings entirely made up of register like panels. Systems of this sort require special types of grease vapor treatment., and generally would only be possible with a suspended ceiling below a significantly higher hard ceiling.

Overall, I would not do this with just a flush set of baffles or other filters at the ceiling. Offhand, I would instead use a two-foot high commercial like hood set with its entry aperture at seven feet. This would provide both head and sightline clearance. (I'm not sure how much better it would look than a lower conventional island hood.) The hood size would be smaller than for the 9-ft case, and so would the necessary hood and MUA flow rates. Drafts would be less of a problem.

A wall location, of course, simplifies the entire configuration.

Full disclosure: I have the largest Wolf Pro Island hood at 34.5 inches above two induction devices under an 8-ft ceiling.