Sounds like 30" high is the right height. You probably want to stick with that, unless it creates a problem for you. Does it? Will you hit your head on a 30" high hood while cooking? Are you worried a hood that low will obstruct your view of others? If you're not sure, it can help to make a cardboard mock-up of the hood, marking its position relative to your range.

Centered over the range is a good plan as well. [This assumes your hood draws air equally efficiently across its entire catchment area. If not, place the best draw above the most smoke.] So, if burners producing the most smoke are at the back of the range, its less of a problem to move the hood back a bit -- if not, don't. Can you live with not being able to cook (produce smoke) using your range's front burners? How about when you open the oven door?

You want your hood to capture whatever rises up from all sources of smoke. An island complicates this somewhat, because air currents around the island cause drafts that nudge smoke away from its vertical path. Making the hood slightly larger than your range, as you plan, is particularly important with islands (especially if open windows, movements, or heating/cooling vents affect smoke as you're cooking).

That said, you can buy hoods that move up and down -- but they're not cheap. If you really must set your hood a little higher than 30", perhaps a larger hood (~40", rather than 36") could compensate somewhat.

thanks for your thoughts chac mool. I am fine with 30" height but my husband is 6'2" and he worries that the hood will bump his head and will obstruct his view of the burners. The range is really in a penisula rather than an island BTW. We were thinking of setting it back 1.5" from centered over the burners front to back and at 31" over the burners to give him a little leeway. The hood would be centered over the burners left to right. Does that make sense? Will that be a small enough change to not compromise the ventilation system too much. Admittedly it does make it less effective for the oven.

I would also encourage you to do the mock-up and try it out. You don't want to have something overly annoying or that causes bad backs or necks in the long run. Capital says that you can put the hood up to 36" above the cooktop in the use/care guide. It seems that true commercial kitchen hoods are 4 feet above the cooktop, or so, but they have a larger capture area to compensate.

I also question the way Vent-a-Hood is calculating loss of efficiency with increased height. It is not a linear function, as we say in mathematics. I'd be more likely to believe that you loose 10% of efficiency with each inch but you don't add them together, you multiply. So 1" up would be 90% of the efficiency and 2" would be 90% squared or 81% efficient and so on. 6" would be about 53% (.90 ^ 6) not 40%. A 40" high hood would be about 35% efficient as opposed to 0% when you add.

I'd say keep it as low as you can without anyone having to stoop to avoid hitting their head.

I'd go farther, although magic lungs may do magic flow patterns. The flow velocity from a hood aperture, assuming relatively uniform flow into it, is plotted in the ASHRAE HVAC Handbook (page 30.4) for a hood of dimension W by 3W. The flow velocity drops very fast as one moves down below the hood. At the center W inches under the hood, it is only 20% of that at the aperture. Most hoods, even at 30-inches above the countertop, are smaller than 30 inches deep, so the flow rate at the pans is more likely to be in the 10 to 15% range. So, if you move the hood up a bit from, say 1.25W to 1.6W, the velocity will drop from 15 to 10%, and you have lost 33% of the velocity.

Fortunately, having high hood air velocity at the pan surface is NOT how we capture the cooking effluent. The hood air velocity could be zero there. The effluent rises on its own up to the hood at velocities a residential hood would not generate, and the hood will capture it if the hood overlaps the expanding cone of effluent. If the hood is undersized, then it will capture a lot of it, with the amount lost dependent on undersizing and, somewhat, to the air velocity at the hood edges relative to that of the effluent (which is typically around 3 ft/s).

To repeat, effluent is not sucked up from the cooktop because it is hot and rises up on its own. If one needed that level of performance, e.g., for immediately capturing the odor of fresh cold cilantro, the actual air flow would have to be of the order of 6000 cfm for a six square foot aperture hood two feet above the surface. Fortunately I like cilantro, so the actual 1000 cfm I likely can achieve doesn't need to be boosted to an industrial level.

Using cardboard cutouts or drawing an elevation view of the cooktop and cook, determine the height that avoids injury and allows good sight-lines into the rear cooking zone. Solve the capture problem with a large enough hood. If necessary, space it out and put a gap filler at the back. Let the more modest residential flow rate solve what it is supposed to solve -- containment -- what has to happen to keep the effluent from bouncing back out of the hood.

Whatever effluent misses the hood will eventually be removed as the air is replaced at the hood flow rate, although some grease and water condensation onto surfaces can occur before it is removed.

kas

So helpful!!! Thank you very much. So if I wanted to compromise would it be better to move the hood higher rather than back, right?

You all are the best!!

It should be best from the point of view of maximum capture to center the hood over the burners. If there is a wall behind and centering leaves a gap at the wall, then adding a piece of stainless steel to to block the gap and direct the effluent into the hood is desirable. Having cabinets on the sides is desirable. Where none of these are present, as in an island or peninsula configuration, one normally uses a larger hood and more flow rate.

Optimality in capture and containment is not necessarily associated with optimality in aesthetics or cost. Ultimately, the consumer has to balance these in selecting a solution.

Perfection in capture is unlikely, in any case, as motion around the cooking surface will cause disturbances in the air that in turn causes some effluent to be pulled out of the path to the hood. So a busy kitchen will not be an odor free one. I would conjecture that kitchen ventilation is like high fidelity systems: Ever larger increases in cash yield ever smaller improvements to the previous baseline.

kas