Lower is better so if you can mount it at 30" without banging your head, you'll be better off. Generally it's recommended to have the hood 3" wider on each side particularly for an island hood. If it's a wall hood and you have cabinets on both sides then, from personal experience, I don't think it matters as much. I have a 30" hood over a 30" range and the cabinets on the sides do not get greasy. I personally think depth of the hood is more important. My hood is only 19" deep and the outer front of the hood does get a little greasy. It's much better to have the hood fully cover the front burners - at least 24" deep and ideally 27".

Thanks weissman. I was just jumping back to the thread since I realized that I left out some info.

This is not in an island and will be flanked by uppers. These uppers should be approx 18-19" off the countertop.

I see you say that 27" deep would be better than 24". I thnk I also read that you stirfry alot :). If I got that wrong sorry, these posts all seem to blur together sometimes when you are reading hundereds of them. But is it still 30" if the depth is increased to 27".

We have 36" Wolf range, the top of which should be pretty much the same as a range top. Hood is VH, 42"X27," wall mount with flanking cabinets flush to the hood. I can't speak to smaller dimensions as this is the only hood this size we've had. I agree with the above poster about 27" depth being more effective (although in our case it also allowed warming lights). We stirfry and panbroil and you can see smoke roll up by the outer edge of the hood and then be drawn up into the hood. The width is a trickier call: we have a high riser with a shelf and the shelf distorts the air flow, especially when we run the grill. My guess is a 36" hood with the shelf would exacerbate our airflow distortion. We're about 30" above the range top, but the range is about an inch higher due to some floor issues. I'm 5'10" and no head bumping.

I do stir fry a lot. 27" deep and 30" above cooktop would be best but if there's a risk of bumping your head then you can raise it a bit. Most hoods suggest 30" to 36" above the cooktop. As you raise the hood, it decreases the capture area a little.

Todds - you didn't ask this, but I'll just butt in and say that while you are working on your kitchen reno and hood, make sure you install appropriate sized duct to the hood. It should be 8" or 10" diameter duct. If your duct's diameter is smaller than that, your hood will be noisier (more air being forced into smaller space creates turbulence which = noise), and it will be less efficient. Also, there is less of a selection of exhaust inserts that will fit a smaller diameter duct. Most of them specify 8" or 10", though you can find inserts with 6" duct requirements, but there are fewer of them.

The desired hood functions are capture and containment. Undesired functions include blocked sight lines to the cooking utensils and cranial damage. These two classes of functions are in opposition, leading all hood designs to be compromises when we factor in aesthetics and cost.

To answer the original question: Effluent leaves the cooking surface of the pan or pot and expands as it rises. Expansion angle and rise rate depend on several factors such as heat input and burner type as described in published studies.

As an aid to visualization, imagine a virtual point source below the cooking surface that emits effluent over a total angle of 45 degrees. The distance to this source below the cooking surface is such that the edges of this effluent cone intersect the edges of your pan. Now follow the expanding cone upward. The hood should capture all of this, if possible. This is the basis for suggested hood overlap of the cooktop, but trapping by cabinets and edge effects that depend on flow rate also allow some slight "cheating" on hood area under some cooking conditions.

Note that it is the hot cooking surface (and for gas ranges the combustion flow area around the pan) that the effluent angle should be measured from, and not the edge of the appliance.

Follow the angles upward to see how large the hood should be as a function of how far up it is mounted. Using a sketch, examine candidate hood heights for good cooking visibility and head interference when bending to reach a pan.

kas
"Off to be the wizard, ...."