Here are some general suggestions:

Ideally, the hood aperture should encompass the rising and expanding effluent. Where not blocked by the back wall or side cabinets, the expansion angle is within 25 degrees of a vertical line over the pan edges. This expansion can be sketched and bump-out vs. height can be evaluated. If the hood size turns out to be impractically (or unaesthetically) deep, reserve the most noxious cooking for the back hobs.

Ideally, the hood aperture should be high enough to not hit one's head on when bending over the stove, or blocking sight lines into pots, but not higher than necessary. A side view stick figure scale drawing helps here also.

CFM have to be sufficient to remove the captured air from the hood plenum (containment). This requires an air velocity equal to the rising effluent velocity (3 ft/s) over an area that is roughly equal to the aperture area at the baffles. (Some slack here can be tolerated depending on the hood baffles' aerodynamic details.) (Hoods one can stand to be near without ear protection do not suck up effluent from the cook top.) Area (sq. ft.) times velocity (ft/s) times 60 s/min yields CFM. Fans need to have zero static pressure CFM higher than this value by some factor dependent on baffle loss, aperture losses, ducting losses, and fan curve factors. Plan on 1.2 to 1.3 to be safe.

Various CFM rules of thumb (or nose) (or grease accumulation) may be found in various references.

As an exercise for the student of ventilation you wish you had, read the references that may be found at my "clippings."

Neglect not the issue of make up air. MUA is required in many jurisdictions when the CFM are greater than some value. This is a safety issue with combustion appliances. Search MUA on this forum for a second tranche of reading.

kas

Ummmm, whut????? I know you mean to help but you lost me on "Area (sq. ft.) times velocity (ft/s) times 60 s/min yields CFM". Would a wood hood bumped out to the distance of the front burners suffice (24")? 6 inches beyond the upper cabs looks ok but anything more than that looks awkward in sketchup. I can go up to 9 inches in height without smacking my head into it. Would that be to high of a hood? If not, do I mount the insert to the top of the upper cabs or just above the wood hood? Thanks for your help. Dude, you know your stuff, it's just that your verbage and math is beyond my comprehension.

This post was edited by halfwaythere on Sun, Dec 2, 12 at 1:56

One purpose of math is to compactly describe things without a lot of verbiage. You may wish to view multiplication as multiple additions. I don't know how to get any simpler. But I went to grade school long before Timothy Leary promoted self lobotomy, so what has been taught in the decades since to replace math is not known to me.

The insert should be inside the wood hood such that the wood is not directly exposed to potential fire. You may be able to download the installation directions for some manufacturer's insert to see how it is done. (Wolf has on-line installation manual pdf's, I believe.) Or discuss your needs with ModernAire or other potential source of your insert.

For description purposes, it is best to measure hood height (bottom of the insert and surround) by its height above the cooktop surface rather than relative to the cabinets. Typically, 30 inches is the minimum for fire hazard, and beyond 36 inches the required hood size for good capture begins to become inconvenient.

If sketching doesn't provide the insight that you need, then try mocking up the hood with cardboard to see what works for you.

kas

Call ModernAire and have them design a custom insert for you if you've already decided on the externals of the vent. Otherwise, check into their 36" inserts and have the carpenter build the hood around the insert. (That is generally the way it's done, as it's less expensive to use standard ventilation products and custom wood products than the other way around.)