We have an insert within a custom made box and after looking at a bunch of options at the time, it seems like your only option is to come up vertically and make an immediate 90 to the back wall. With an 8” duct and the thickness of the insert, you’re looking at something no longer resembling a floating shelf.

Thanks! - rear exhaust will be two rectangular ducts equivalent to 8" round duct, so seems we can keep the low profile.

It will need to be 8”-12” tall, minimum.

Do you actually cook in your kitchen? How often? What styles? What kind of range/cooktop will this be over? How important is indoor air quality and health to you and your family?

Containment area (empty volume in a hood enclosure) is critical to hood performance. You also need to think about grease dripping from baffles that aren't sloped enough if you prepare foods that produce greasy effluent.

More: https://www.houzz.com/discussions/5161173/hood-faq

Hear you on containment area. 1500cfm external blower, so hopefully there's minimal fumes just lingering in the buffer volume. To your other question, strongest burner is 18kbtu and I do sear stuff when the kamado isn't available.

"Second, you want that flow rate fairly even across the aperture because you won't have any airflow to waste. This requires that the baffles effectively gag the air flow relative to the hood height flow. Otherwise, the space above the baffles has to get higher"

• excellent point - at least it was something that occured to me in trying to optimize this:)

Note that higher CFM does not generally make up for too small of aperture nor too small of containment area. You're dealing with very hot and heavy effluent that rises and does as it wants and does not easily bend to follow airflow. For example, a 27" deep hood requires 7x as much airflow (CFM) to perform the same as a 30" deep hood.

opaone: I trust you mean in the sense of capturing effluent headed upward and outward such that it would miss a 27-inch entry aperture boundary by 3 inches.

Weifei Zheng: A four-inch high shelf hood really won't have a significant buffer volume (assuming you mean volume below the baffles); the baffle assembly itself might be 1.5 inches thick and will have to be low as possible. The air flow in the volume above the baffles will not have any lingering fumes. But to get the flow at the baffles to be roughly equal across the baffle area, the pressure inside this type of hood has to be low relative to below the baffles, and this pressure loss moves the user up and to the left on the fan curve, reducing volumetric flow rate. Hence the suggestion for zero static pressure rated flow rate to be twice the required operating flow rate.

(Also note that I did the calculation for a nominal 24-inch entry aperture (front to back); a 27-inch will require 1/8 more air flow to keep the velocity at 90 ft/min. Maximum stud space ducts of 3.5 x 14 might be advised, along with nearly matching slots in the hood rear.)

@kaseki, yes. An engineer at either Captiveaire or Accurex told me that for a 27" hood to perform the same as a 30" that it would need approx 7x the CFM's. This varies though by burner location, pot size, pot height, etc but as a general rule of thumb that's what it is. IIRC, for a 30" deep hood to perform the same as a 36" it only requires about 1.7x CFM.

BTW, what you are calling Buffer Volume is what they call Containment Area - it contains bursts of effluent until it can be exhausted by the blower. In effect it averages out the bursts so that the blower can keep up.

I was just repeating the term used by Weifei. Nothing is going to linger in a near zero volume. For that matter, not much lingering is going to occur in an enormous commercial hood containment area if the volumetric flow rate is such that the entry aperture flow velocity meets the requirements of Table 4, page 10 of the Greenheck Guide, even if the baffle area far up inside is significantly smaller than the entry aperture area. I prefer to think of the action as momentum reflection insufficiently entrained by the air flow than lingering.

great analysis! If I can synthesize the concepts from kaseki and opaone as it seems they pertain to this proposal: Ahigh flow rate (2x+ typical) will be required for this relatively inefficient setup to function, though it seems like based on the Greenheck guide plot at least, the proposed cfm may be sufficient. Sizing the hood to be even wider than the range is not an (aesthetic) design conflict, so that's one way to improve performance.

Normally (in fact always) the actual flow rate of a blower in situ is less than the zero static pressure flow rate. I usually assume (where there is no analysis) that the required flow rate should be multiplied by 1.5 to find the rated flow rate that the blower should have inorder to cover all the sins of the design. In this case with narrow ducts and narrow hood height and transition losses at the slots in the back connecting to the ducts and who knows what in the duct path and indeterminate MUA, I have suggested a 2.0 factor. This is, therefore, a factor of 2.0/1.5 = 1.3x higher than usual (and maybe typical). Greenheck Guide gives the flow rate at the entry aperture, but the blower required is the result of analysis of the hood, ducting, and MUA vs. the blower's particular fan curve (which may be affected by fan belt sheave ratios in some configurations).

The hood should be wider than the range and (for a wall hood) also deeper front to back so as to overlap the rising and expanding plumes from the front burners and the pans thereon.

"Nothing is going to linger in a near zero volume. For that matter, not much lingering is going to occur in an enormous commercial hood containment area if the volumetric flow rate is such that the entry aperture flow velocity meets the requirements of Table 4, page 10 of the Greenheck Guide, even if the baffle area far up inside is significantly smaller than the entry aperture area. I prefer to think of the action as momentum reflection insufficiently entrained by the air flow than lingering."

With a properly sized & designed containment area (eg, commercial hood or properly designed consumer hood) it lingers long enough though. The difference in a burst of effluent remaining within the hood's containment area until it is exhausted and that same burst of effluent rolling out from under the hood and spreading throughout the room and house is likely only a second or three - but that second makes a big difference.

It's these bursts that are critical to capture, contain and exhaust as they could make up perhaps 90% of the odors, grease and steam.

Hmmm. I understand what you are describing, opaone, but have to note that I've never seen a plume transient that strong from surface cooking done by me or my DW. Even the Cooktek wok hob with a plate of almost cooked components dumped into it for final heating generates a steam and grease blast that is entrained in my Wolf hood with its modest containment area. This would not be true, of course, with an asian restaurant wok hob running 100k BTUh and the same hood. Perhaps you and I must have different food preferences.

On the other hand, opening a range oven door when broiling could easily overwhelm a hood over the range. My ovens are on a wall so that issue is essentially uncorrected by my cooktop's hood.

Also, my first sentence in my comments above applied to a 4-inch thick hood with baffles flush to the bottom. There is no containment area. As for your hood, I don't see what you could do on a stovetop to fill that volume with a transient. Please fill this gap in my knowledge.

kas