Thank you @kaseki and @billy_g for your helpful comments and suggestions! I am really impressed with the hood that you designed and had built @billy_g. I have a couple of questions for you relating to that. Did you install a MUA system? And where did you get the removable grease cup? Is that something that can be ordered online?

Yes, you can get a grease cup from restaurant supply houses or on Amazon: Commercial Kitchen Restaurant Duty Canopy Hood Grease Cup- Removable Keyhole Mount 4” Deep (Large Capacity Grease Cup) https://a.co/d/cZ7VbzU

Thanks! I might end up having my GC get his metal fabricator to build a hood for me as well using your drawing but I noticed that there's a note that says See drawing re angled part for baffles. Is this a separate drawing that you can include as well or is it just referring to the angled part that you have drawn to hold the baffles?

Here you go:

Thank you so much for all your help! Have you been able to use your hood yet?

@billy_g, nice job! What baffles are you using? (Edit: Just noticed the note on your drawing that I missed earlier).

@ksanchez9 I have not used the hood yet but I like listening to it! Gas for the cooktop will be connected next week.

@opaone thank you for compiling the relevant information and sharing your experience and journey with your hood. This was key!

@billy_g hopefully, you've had good success with your hood now that your gas should be connected. How is the 1400 cfm blower working? Do you think you could have gone with a smaller one? I am still trying to locate a company to build one for me but I think that I might have some leads. I did have one more question about where you located the switches for your light and fan as I didn't notice them on your drawings.

We're not moved into the house yet (long story) so I don't have an update on the blower. I suspect you could go with a lower CFM blower for a 42" hood (and 36" high BTU cooktop).

I located the switches on the kitchen backsplash - not ideal but not a problem, and in this case better than mounting them in the hood and blocking the airflow. You also could mount the electrical junction box in a wall cabinet and have the controls under the cabinet but that's kind of awkward...

Reminder: Residential hoods with sparse reservoir volume below the baffles need air velocity across the entire entry aperture for the highest velocity plume cooking (up to 1.2 m/s), whereas with Commercial hoods having large reservoir volume below the baffles, the hood averages the plumes and can generally get away with lower average air velocity. So @opaone can sear or wok on a burner and get away with lower CFM with his quasi commercial hood than I can with my Wolf Pro Island hood. If he is searing on all burners, however, the total plume CFM (sum of plume diameters at hood entry times plume velocities at hood entry) is the minimum flow rate he has to use, which will be approximately the same as I have to use (90 CFM/sq. ft. = 90 ft/min) even when I am operating only one hob.

See the table from the Greenheck Guide below. I operate at the "Medium" flow rate for hot oil cooking, where Opaone can usually operate at the "Light" flow rate. This relation is true proportionately when cooking with lesser plume loads. [Click image to expand.]

https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf

This relation, however, doesn't continue to near zero plume velocity (e.g., cilantro watching). The commercial hood at 7 ft height is more susceptible to cross drafts and at very low plume velocities may well have to operate at a slightly higher CFM than the residential hood at 6 ft. depending on the conditions. In any case, minimum operational CFM is likely driven by baffle grease extraction requirements than capture and containment of low velocity odors.

@billy_g this looks like a fantastic solution, and I'm thinking I might borrow your drawings (and parts suggestions) to see if I can find a fabricator willing to build something similar. Definitely do post updates on your project!

One complication for my kitchen, I think I'll need the duct to open from the rear rather than the top. It's a direct 10" round duct that just goes horizontally right outside near the top an 8' ceiling. I think ducting from the top would require placing it too low, unless I were to do a version with a lot less height (but then the baffle angle would probably have to change). I'd be doing an external blower on the outside wall of the house. Given that the duct is otherwise very short and free of bends, does anyone think rear ducting would negatively impact the performance of this design?

I currently have an Elica over which performs pretty poorly over a 30" Capital range, and I've been mulling over what to do about it for almost a year now.

It shouldn't be a problem to duct from the rear. It would lower the noise if you have a bend rather than a straight duct run... is there room for a duct silencer in the straight run?

I’m pretty sure there isn’t room radially, but also the “run” is less than a foot. I think it at least shouldn’t be worse than my current setup, an internal blower with fixed settings that can’t run lower than a (claimed) 300 CFM? Unless the box would create some sort of resonance that amplifies the noise or something.

But I’m also hoping high containment volume would allow most everyday range use to work well running below “300”, as in the “light” column in kaseki’s chart above.

Ah, I see. Your goals make sense to me.

Maybe @kaseki can chime in with his thoughts.

For additional context, I had for a while been considering the Prestige High Capacity hood that was discussed in another thread some months ago (since it actually is deeper than other residential hoods and has a design that avoids wasting capture area space on a front light bar as most do), but I’ve been hesitating because it’s still not quite what I want and would either be a bit too high in my kitchen if rear ducted or mounted at a head-bonking height given it’s depth if top ducted. Whereas this solution seems like it would be way more performant, and I could get the height right.

Booonnngggg. I prefer the gong over chimes for a wake-up call.

Not sure which thoughts need to be expressed, but I will note that the airflow requirement that I usually recommend -- 90 ft/min = 90 CFM/sq. ft. (see Greenheck method "Medium" column in Fig. 4 in an earlier message above) -- is based partly on the medium column value (note that Greenheck's values are for commercial hoods with gigantic reservoir volumes below the baffles, not residential hoods with barely any) and the idea that good entrainment of the rising plume effluent should be achieved if the baffle gap air velocity is commensurate with the maximum plume velocity (3 to 4 ft/s). So my recommendation is for residential hoods performing "Light" column tasks.

If the user intends something extravagant for interior cooking, such as operating a char broiler, then significantly higher air flow is likely needed, and it may not be possible to fully collect the plume without a more commercial-style hood configuration.

At the low end of the air flow regime, flow rates for minimum rising plumes should not be below the speed that allows the baffles to centrifugally extract grease, or below the speed that ensures that flow into the baffles is not easily disturbed by barely noticeable turbulence or drafts. This is likely why the Wolf control in my hood has a minimum flow rate.

@opaone I have read all of your posts and comments... thank you for being so informative on this topic! On a sidenote: I happen to follow your designer and builder separately on instagram and have pinned many photos from your house as inspiration for our new build. I went to your Wordpress site for the exhaust hood FAQ's and realized that it is in fact YOUR house! What a neat moment! It is beautiful.

A few questions re: hoods.

1) I am waiting on a reply from our local Accurex reps re: doing a 42"x30" residential hood for a 36" gas range (either Viking 5 Series or Bluestar RCS)...its been a few days and nothing, so I'm trying to formulate another path forward. In lieue of Accurex, my backup is the Wolf Pro Wall Hood which seems to be cautiously recommended by @kaseki as the best residential option at a 42"x27" depth mounted at 36" above our range (upper maximum in their specs--I am a tall person). I mainly make mac n cheese and scrambled eggs for our kids... any high heat cooking I will plan on doing on the back center burner(?)... I don't think I ever use more than 2-3 burners at once and even that's pretty rare .... knowing our lifestyle / cooking needs... does this seem like an adequate compromise?

I don't want to throw almost $3k down the drain over a completely inferior solution, but I'm not doing anything that elaborate in the kitchen. (Also if there's a cheaper option, please share!). I looked at Prestige, but their website gives me the impression that they are out-of-business / not reliable.

2) We are building a home in New Hampshire... I know your house is in Minnesota. Our builder is advocating a 6" duct mainly for insulation purposes. He is fine going larger, but has warned us that insulation and cool air seeping back into the kitchen is a potential tradeoff. The specs for the Wolf require a 10" duct. Is our hood going to become a vehicle for ice cold drafts during our Northern winters? Any thoughts around countering this potential issue?

3) New Hampshire building code is 400CFM before requiring makeup air. We have been told we don't need makeup air... some reasons being: our house is very large (4600 sq. feet) and very open. We have high ceilings (10'). And like you, we insisted on a wood-burning fireplace, because there is nothing better on a cold winter night than a wood-burning fire... but with that will come more drafty air into our house. Due to the "capture area" of our 42x27" hood, can we stay within the 400 CFM range, avoid makeup air, and still have good indoor air quality.

Follow-on to this question. It sounds like ideally we would want to do an inline blower, however the minimum Wolf has is a 600CFM option. My understanding is that a 600CFM inline blower is not actually operating at that capacity. Is there a way to calculate it's actual operating capacity (i.e. would it be under the 400CFM code threshold).

I will push for makeup air if it is irrevocably necessary (and it sounds like it is virtually always advocated for by you and @kaseki). I'm just wondering if any of the specific factors to our build and lifestyle are relevant.

Thank you in advance for any feedback you can share!

Well, I live in NH, and can provide some climate-relevant comments, at least if you live south of, say, Plymouth.

I should note that my Wolf hood (actually built by Independent for Wolf in ca. 2008) is a 'Pro Island' model hood. I believe all of the Wolf hoods that are of the canopy type -- wall and island -- will be suitable for cooking ventilation if large enough and provided with sufficient flow rate. I have no other styles of Wolf hood to directly evaluate.

For those who have reached this point via tl;dr, a brief summary follows without much justification. First, it is necessary to determine requirements and then select among candidate hood systems that meet those requirements.

• For capture of rising and expanding cooking plumes, the canopy has to overlap the plumes at the height of the canopy. For 36-in hood base hight over the counter, this is about 3 inches wider than the cooktop, as well as deeper.
• For containment of hot cooking -- defined here as hot oils at the smoke point, searing, wok cooking, and other combined moisture/grease plume producing cooking -- an air velocity at the hood base of about 90 ft/min is desirable with residential style canopy hoods with small volumes in the reservoir space below the baffles. This directly implies 90 CFM/sq. ft.
• Typical blowers in typical pressure loss operating conditions will likely move only about 2/3 of their rated (zero static pressure) flow rate.
• So, take the intake area, multiply by 90, further multiply by 1.5, and the result should be in the ballpark of the required rated blower flow rate (CFM). This is likely to be higher than 400 CFM. Cooking content, induction vs. gas, and other factors may allow some reduction of the calculated requirement, while poor MUA can increase the rated requirement, but I wouldn't drop very much if you want to ensure clean interior air and no grease on the house surfaces.
• Duct size should limit full power air flow to between 1000 and 2000 ft/min.
• No air is removed from the kitchen that isn't resupplied. Insufficient supply just causes the blower to move less air. The resupply is called make-up air (MUA), and it can be deliberate or accidental (through the walls, say). Deliberate MUA can be passive (no blower) or active (with blower). Accidental is undesirable for many reasons, and just because there is a limit at 400 rated CFM doesn't mean MUA should be ignored.
• How well the MUA needs to keep the kitchen pressure close to that of the exterior air depends on presence or not of combustion appliances that can be back-drafted at some household pressure. Back-drafting is hazardous.

So by example 42 x 27 is 7.9 sq. ft. calling for about 700 actual CFM and around a 1000 CFM rated blower. In a 10-inch duct, 700 CFM has a velocity of 1200 ft/min, so I would not go smaller.

Also, please note that with a roof damper and a damper above the hood, minimal cold air will be drawn into the house when the blower is off.

I space allows, a silencer (Fantech LD-10 for example) can be placed between hood and blower for a considerable reduction in noise.

If you use passive MUA because of lack of combustion appliances, you will not be able to run the fireplace when the blower is operating at higher flow rates. For this an active MUA is needed, such as Fantech makes.

_________

P.S. A house as large as the Pentagon would eventually drop in pressure with an externally vented hood unless air was replaced. House size doesn't matter except for providing more leakage paths that you should avoid using.

P.P.S. Wolf answers their phones and provides technical data. Otherwise, they are one of no doubt many suppliers of quality hoods that can provide good capture and containment when properly specified. For example, I strongly suspect that ModernAire knows what they are doing, even though I have never seen one of their hoods in person. Similarly, ABBAKA, to name two.

Thank you @kaseki ! I did not realize you were in NH too, this is very exciting!

I have the Wolf pro wall hood earmarked for its 27" depth (https://www.subzero-wolf.com/wolf/range-hood/42-inch-pro-wall-hood-27-inch-depth), and it sounds like "best practices" means I need makeup air :)

Re: your point: "For containment of hot cooking -- defined here as hot oils at the smoke point, searing, wok cooking, and other combined moisture/grease plume producing cooking -- an air velocity at the hood base of about 90 ft/min is desirable with residential style canopy hoods with small volumes in the reservoir space below the baffles. This directly implies 90 CFM/sq. ft."

I will virtually never be cooking hot oils at their smoke point :) Is there a CFM rule of thumb for boiling water, etc?

Last question, regarding the the 1000 CFM blower. I am under the impression there is some CFM loss depending on the location of the blower? Is the 1000 CFM rec for internal, inline, or remote? (or maybe my premise is incorrect). Thank you!

Last question first. We can model a blower (motor, fan, housing, etc.) as a device that is ducted from output to a point of resistance and then back to the input. Real ducts have losses (friction, turbulence, etc.), hood baffles have losses (change flow direction twice), and transitions such as in the hood have losses. WE call these pressure drops "pressure loss." It is the loss around the loop from cooktop to outdoors to indoors at the cooktop that add up to the total pressure loss. To first approximation, it doesn't matter where in the loop the losses are, and correspondingly, it doesn't matter whether the blower is in hood, in duct, or at a house boundary. In reality, different blower types may be differently sensitive to duct effects close to the blower, so position relative to an elbow, say, might have an effect. Also one might have one of each type with the same zero static pressure rating, but with different fan curves relating flow to pressure loss. Other than using good design practices, these anomalies should be accounted for by the 1.5X factor I introduced above.

If you are generating pure steam, only activities such as boiling down maple syrup would likely over humidify the house, but in any case, if the steam upward velocity is much less than 4 ft/s, then a lower value than 90 ft/min may be sufficient.

However, normal cooking entrains more than water into the plume, and these components can leave residue on surfaces, along with odors. Poaching fish might not require a very high air velocity for containment, but you certainly don't want to fail capture, which can be influenced by air flow rate. A low velocity plume might also be a somewhat spreading plume, particularly when turbulence from human movement and drafts interfere with the rising plume.

The only other consideration that occurs to me at the moment is resale where a hood functional for the full gamut of residential cooking is usually desirable.

Thank you @kaseki this all makes sense, I appreciate you taking the time to respond to these and tailor your answers for those of us that are less engineering-minded!

@billy_g and others - For those making a custom commercial style hood, did you add an "air gap" on the back? I see some hoods like those from NAKS/Hoodmart have an air gap on the back. Is this necessary? I am in the process of finalizing a design for something very similar to what billy_g built (thanks for the inspiration!) and this is the last detail I'm trying to figure out so would appreciate any thoughts and advice!

I'm hoping that @kaseki is still monitoring this thread as I have a question. Most pro style hoods, such as the Wolf have light and/or controls along the leading edge that significantly reduce the deep capture area of the hood. In comparison the VAT designs have a much narrower band. Not as narrow as commercial hood, but still the difference between Wolf/Thermador and VAT is several inches. However what I don't understand is how significanty, if at all this affects capture. Does performance fall off dramatically if the first few inches of depth are modest or is the effect minimal?

This is not an idle quesiton since both aethetically and with regards to build quality we prefer the Wolf hood but we plan to use the grill that came with our 48" Wolf range and if smoke is going to spill then we'd go with the VAT.

Just so we're comparing like to like and only discussing the single variable of the effect of the design of the leading edge on just capture (and not containment volume), I'm assuming a VAT with baffles and an external blower an option they offer but prefer not to talk about.

Just as hood air flow velocity rapidly drops with distance below the hood (somewhat dependent on whether wall or island mounted, side skirts or cabinets, etc.), so too does it drop with distance horizontally at the hood base edges. At the edges, the rate of velocity drop (flow deceleration) is higher than vertically below the hood such that high velocity plume "rays" might not be deviated into the capture area at the edges. Fortunately, plume ray velocity drops with angle from the pan edge, so this is partially compensated. One can measure air flow across the edge boundary, so, qualitatively, a claim can be made that the hood entry aperture extends to the exterior dimension of the hood at its base.

Ultimately, capture and containment depend on the hood flow entraining the rising plume into its flow, and for a given configuration and cooking project, this depends on sufficient air flow rate (CFM). Whatever configuration is chosen should be analyzed against all the pressure losses from cooktop to outside and back to the cooktop via whatever MUA path is present.

Note that the internal configuration is relevant. Somewhat similarly to light striking mirrors, the plume "rays" can multiply reflect with only modest momentum loss and escape a hood with insufficient air flow. This is similar to trying to concentrate light with a cone reflector and failing to recognize that the Lagrange Invariant applies to attempts to concentrate rays.

Commercial hoods tend to have high reservoir volume below the baffles, which also tend to be lower in area than the hood entry area. They internally allow typically only one or two bounces as they narrow at the top, and can get away with averaging the plume CFM over the baffle area, thereby allowing a lower CFM than residential hoods that are necessarily restricted in height.

VAT? Do you mean VaH for Vent-a-Hood?

My Wolf Pro Island hood has an internal front-to-back depth of 26 inches, an external front-to-back distance of 32 inches. Given that the light bar has no grease build up, whereas my recent baffle cleaning revealed significant grease in the grease trays, I would argue that the air flow was sufficient to keep down plume impingement at the light bar to a negligible amount, implying effective capture since the air flow there is inward. YMMV. The greatest plume deviating factor in my environment is the cross draft possible from a nearby mini-split air conditioning unit on an interior wall.

Cooking over the full extent of a grill area would suggest a plume base equal to the grill area, and this might well require a hood that provides more front-to-back distance than is considered normal. I doubt that a VaH Magic Lung® with narrow front edge will have a flow rate there higher than a Wolf achieves under the lamp bar with a 90 ft/min average air velocity under the hood.

If you would calculate for your configuration the angle from vertical between the grill front cooking edge and the candidate hood edge at the intended hood height, I could look into the "Finnish" paper for the plume angular shape that resulted from some plausibly similar case. Is the grill lamp heated or gas flame heated?

I very much appreciate your offer to look at the specifics of my situation but first if I can I'd like to see if you would be willing to help me a bit more to understand how the geometry of the interior of the hood affects or doesn't affect just capture. I'm putting aside for the moment the effect of containment which I know has been the subject of much discussion.

Assume the flow is as you specify being necessary. What I'm grabbling with is whether capture performance is significantly affected by whether the hood is deep vs shallow along the front few inches of the leading edge of the hood.

From your response I'm taking away the answer is no. If I'm understanding correctly then, again putting aside the effect of containment volume, would that mean that two hoods identically installed (same height, coverage, air flow) one of which was flat with the baffles on the bottom and the second with recessed baffles would perform the same as far as capture?

Perhaps I'm thinking about this the wrong way but I'm trying to separate the effect of containment from capture. The reason is so I can understand how hoods such as Wolf or Thermador that have that wide band at the leading edge would perform compared to a design where there was a very narrow leading edge.

If the effect is minimal then I'd move on to comparing one of these moderate volume hoods with one that was larger volume, such as the Vent-a-hood only based on the difference in containment.

I posed the question, and my example of an entirely flat hood, inorder to wrap my head around the physics not because I'm planning on installing one.

The grill is gas heated. The hood is the Wolf PW482718. Wolf doesn't specify either the distance from a reference point such as the back of the stove to the front of the grill nor from the wall to the front lip of the hood so I'll drop by the local dealer to confirm these dimensions and post the number you asked for.

Thank you both for all your posts over the years and the offer of help with my particular situation.

I think of capture as the initial collection of cooking plume(s) by the hood entry aperture. This is a function of the plume(s) size (sectional area), hood entry aperture size, and any effective widening of this size by the actual air flow. For example, side skirts and back walls extend the apparent hood size.

Containment is the removal of the plume from the kitchen, which implies at a minimum transfer of the plume to the exit duct size of the baffles, or past the squirrel cage blowers in the case of VaH Magic Lung systems.

To get from capture to containment requires supporting flow rates and hood sheet metal geometries. To some degree, flow rate (CFM) can compensate for non-ideal geometries. One should keep in mind that commercial hood systems are designed for capture and containment success with the minimum electrical power usage, as this is a big expense for commercial kitchens. As @opaone has commented and demonstrated, commercial geometries allow lower CFM for capture and containment success. This I attribute to baffle averaging in hoods with large reservoir volumes below the baffles. Nonetheless, spillage -- reflection of plume content out of the hood -- can occur with any hood operated at insufficient flow rate.

Schlieren photos of hot air rising from a griddle

In a hood such as the Wolf Pro Island hood, there is a modest volume below the baffles (which are tilted) that provides a modicum of plume averaging, at least for transient plumes. This is greater for my longest island hood covering two induction devices. However, it is still mostly passing plume effluent through the baffles that the plume impinges upon. In other words, containment is immediate after capture because plume rays impinge mostly on the baffles without any reflection from other surfaces. Any rays that strike the central support will be reflected downward but will be entrained by the upward flow. They can't easily reach the hood edges.

In a hood such as the one above, plume components rise up the hood reflecting from the front and back walls until reaching the top where the baffles are to one side, and the front-to-back width is narrowed. The plume will likely churn a bit up there before being pulled into the baffles. Success requires sufficient air flow to make up for the plume's momentum when reflected. The 'Greenheck Guide' Figure 4 suggests the specific flow rate needed to assure containment vs. failure by partial reflection and spillage. (Feet per minute equals CFM per square foot.)

These values are in my view too low for residential hoods without significant reservoir volume, hence my suggestion that 90 CFM/sq.ft. be the goal for residential pan cooking. This value may be marginal for containment of plumes from actual grills. Whatever the goal, it requires higher blower ratings to make up for the fan curve, where restrictions inevitably cause lower flow rates.

So am I correct that the guiding design principle is that the velocity of the air at the face of the hood needs to meet or exceed the velocity of the rising effuent ladden air plumes so that air that make up the plumes doesn't bunch up behind slower air being drawn into the hood by the blower as this would result in some or all of the plumb being forced to move horizontally and spill over the edge of the hood wherever there wasn't an abstruction (such a a wall at the rear of the range)?

Sorta. If we allow the baffle gap space to be 50% blocked (or open), the hood air velocity at the baffle gaps will be twice that at the entry aperture. -- more for narrower gaps. I have thus argued for 90 ft/min at the aperture, resulting in 180 ft/min or so at the baffle gaps, which is in the ball park of the highest plume datum I have: 1.2 m/s at the peak of a plume velocity distribution profile. In any case, no one since I started spouting off here ca. 2010 has complained that my recommended flow rate was too low.

With respect to Figure 4 of the Greenheck Guide, 90 ft/min is approximately the value one column to the right of typical residential cooking, and would roughly correspond to having limited to no reservoir volume to average the plume's velocity distribution over the baffle space. Or, we can just go with intuitive for air flow rationale.

I don't think the "bunch up" model applies. The rising effluent will just save the hood the effort of inducing momentum into the air it is pulling through the gaps. This is not to say there is no turbulence near the baffles, but so long as there is no turbulence below the hood entry aperture causing missed capture this won't matter, methinks.

More difficult is the case I can't evaluate. Just because the 100 ft/min I can generate at my hood is OK for wok plume capture, and for small steak searing capture with some splash blockage degrading plume velocity, doesn't mean that it would necessarily do for your grill operating full blast. I don't know if @opaone does grilling as my copy of his cooking area photo doesn't show an obvious grill.

Hence, one could argue that your blower should be sized for 150 CFM/sq.ft. (Column 4 of Figure 4) IF you have horizontal or modestly slanted baffles providing limited reservoir volume in lieu of the commercial hood design Figure 4 was established for.

Got it. I believe I understand the design process. Multiply the the aperature area by whatever number you feel comfortable with for cfm/sq ft and size the blower system, included derating, to provide that amount of flow.

But what I'm not clear on is what factors affect translating the flow in the duct to the design goal of X cfm/sq ft at the aperature. I can imagine that the velocity isn't constant across the face of the aperture but instead some areas exceed the design spec and others falling short.

Again my fluid dynamics was rudimentary to begin with and has only gotten worse but I'd think that the geometry of the hood would strongly influence this.

For example imagine the two extremes. First a flat plate style hood with for simplicity the duct coming out the middle, My intuition tells me, perhaps incorrectly that the velocity of the air at a position directly below the duct will be significantly higher than off to the side falling to a minimum at the edge of the plate. So short some kind of internal baffles this would be a very poor design.

At the opposite extreme visualize the duct connected to an exceedingly long bullhorn that ends for simplicity with a radius equal to the desired apeture. Ignoring edge effects, which would be small for relatively low air velocities, again my intuition tells me that the gradient for the velocity across the face of the aperature would be small.

I think you can see the question I'm wrestling with. Assuming suitable air flow velocity in the duct, what is hood geometry is necessary so that the velocity of the air where it is slowest, which I presume to be along the unbounded edges, still meets or exceeds the design spec?

Again intuition says that shallow hoods, like the Wolf are going to have more of a gradient than deeper ones but that doesn't mean that shallower hoods can't meet the design spec across the entire face if they are thoughfully designed. But its not clear they are and that is the heart of the matter.

So while I understand and agree with your calculation it seems that one can't assume a given flow in the duct will result in the required flow at the aperture without understanding the affect of the geometry of the hood.

What, if any light might you be able to shed on this?

Because the baffles represent a significant pressure loss (as a function of air flow, or course), there tends to be some averaging across the baffle space from that fact alone. The baffle restriction dominates any flow induced pressure loss as air proceeds from baffle back to duct entry. (Slim hoods with mesh filters will tend to be more uniform due to the pressure loss at the meshes.)

In the case of Wolf Pro hoods, while the duct to hood transition is not logarithmic horn, or even bullhorn, there is a lot of room above the baffles to keep the pressure behind the baffles relatively uniform, even with some transition losses. My measurements suggest that while the flow rate drops at the hood edges and exactly under the central baffle support (for my island hood), the amount of variation is modest.

The volumetric flow rate in the duct is the same as through the baffles and through the bottom entry aperture of the hood, ignoring slight variations in air density. And by extension, through the house, through the MUA system, and throughout the house exterior. For reasons we haven't yet discussed, but which I have widely noted here, one wants to keep the flow velocity in the duct between 1000 and 2000 ft/min. This will require 10-inch or larger ducts, such that air density is not that different between all points in the system (exclusive of regions close to blower blade surfaces).

My head is spinning with flashbacks from my HVAC classes.

Allow me to reflect back to see if I understand correctly.

The gradient of flow across the baffles is small for the reasons you state. So the model for the hood is a transition from the cross sectional area of the baffles to the cross sectional area of the hood. That's a much more modest change so gradient of the velocity of the air across the aperture of the hood is modest.

Is that correct?

The gradient across the baffles is large, the gradient between baffles and duct is small, the gradient in the duct is higher, but not typically as high as across the baffles unless the duct is very long or too small in diameter. Similarly, the gradient through the MUA path likely includes a filter and heater, so that can be significant. Under some conditions, generally where combustion appliances that can be back-drafted are present, the MUA will require a blower to compensate for its pressure loss.

Hmm. In that case I'm confused. Let's assume the MUA is sufficient (structure has a large door in the basement that opens to the outside when the fan turns on so there is close to zero friction pulling the MUA in) so I can wrap my head around just the issue of the hood as I have trouble with more than one variable at at time.

The gradient I'm referring to isn't across the baffles from the side facing into the hood to the side facing the duct but perpendicular to that from one end of the row of baffles to the other.

So facing the hood as if you were cooking and swiveling your head to the right and then to the left there is a line of baffles along the back of the hood. Are you saying the velocity measured at the face of those baffles is fairly constant or varies significantly as you move from one end to the other?

I'm only referring to the velocity at the face of the baffles.

I thought the answer was its doesn't vary significantly but as I said I'm confused.

Ah. What I thought I said (or at least meant) was that if the pressure loss across (thru) any baffle position was "X" and the pressure loss getting from the duct side of the baffles to the duct was <<X, then the flow through the baffles will be relatively uniform.

The contrasting case of baffle assemblies in the dishwasher would have the flow highest below the duct decreasing toward the hood ends. If the hood section is relatively thin, then without the baffles one has the duct for a hood and the hood for a decorative skirt.

For the benefit of any innocent bystanders who are following along we, or at least I, am trying to determine whether currently available hoods such as the Wolf 27" deep wall hood (PW482718 in 48" wide ) and similar models from Thermador are likely to perform better, worse or about as well as the 27"deep hood from Ventahood assuming both are equipped with remote blowers and baffles. This not the normal configuration for the latter but allows for comparing shallow hoods with deep bullnoses against a hood that is taller and with a narrow leading edge.

Kaseki has shown, at least to my satisfaction. an essential requirement for good hood design is that the air being removed by the blower must have a velocity at least equal to the velocity of the rising effluent air plumes. If it were otherwise, although he describes it slightly differently, the rising plumes would bump into air being drawn into the hood that was slower moving when they need to be traveling at at least the same speed.

The requires that the velocity of the air drawn in by the blower be not only moving at sufficient speed but sufficient across the entire face of the hood including the edges. If it is not then the rising plumes would be forced to move horizontally and spill out of the hood. Since air that spills out cannot be evacuated this is highly undesirable.

The question is whether current shallow designs from Wolf result in a more or less consistent flow across the entire open face of the hood. Absent proof one can imagine that air at edges might be moving slower than at center. If the difference is sufficiently large then the speed of the air along the edges could fall below speed of the rising plume in which case spillage is sure to occur.

As a result the question is how large is the difference in flow rate (speed) is across the downward facing opening (the aperture). My intial model lead me to believe it was likely to be significant. Kaseki pointed out that my model wasn't very good because it failed to account for the dispersion of the baffles and he is absolutely correct.

So I proposed a model with air being extracted through an area the size of the face of the baffles with the flow at any point on this rectangle being approximately the same. Air is entering the hood via the rectangular opening facing downward. This opening is significantly larger in area than the face are of the baffles. The question is whether the velocity of the air entering the hood at any given point falls below the minimum required ie the velocity of the plumes rising from the cooking service.

This would be purely be a function of the internal geometry of the hood. While one could try to overcome any disparity by increasing the amount of air being expelled with the goal of bringing the speed where its below what the minimum its better to first see if improving the geometry of the internal geometry of the hood was possible or practical.

Which gets us to the question that I'm trying to answer. How much does the flow change across the face of the hood in current designs? If the answer is "not much" then we're done. If the answer is "more than it would with a change to the interior geometry" then the question is how much would the improvement be.

Its not clear to me what the answer to that question is. Kaseki mentioned he'd measured the flow at the edges and he didn't see significant falloff. I believe him but because I feel the need for a model that supports his data my continued pestering is try and draw out a model that support his empirical observation.

I'm afraid that some restatements of my writing above are not congruent with my thought, and perhaps with my writing.

(a) I specifically wrote that the air velocity of within the hood needs to entrain the rising plume, and that velocity in the baffle slots equal to the plume peak velocity seems to be sufficient for this (with residential hoods).

It is also true that the hood actual CFM (no matter whether residential or commercial) has to equal the total CFM of all the plumes in play at one time. Typically for suggested flow rates this requirement is met. For one case in the Finnish plume paper that I re-read yesterday, the actual plume volumetric flow was 320 CFM.

(b) I wrote that there was flow across the front (lamp bar) edge. It is likely to be sufficient for the lower plume ray velocities at that angle from the pan edge. I have no data for a grill. It does drop from inside edge to outside edge. It does wrap around. The flow will be stronger for wall hoods than for my island hood. Figure 6, Chapter 30.4, of the ASHRAE Handbook: HVAC Applications shows that sharp edge flow fall-off rate is about 3x that of the central flow, but to the extent that that applies to the Wolf light bar zone, such a fall-off would imply the flow rate at the cook's side edge would equal that which is about 18 inches below the hood entry aperture in the center.

(c) Variation under the baffles is not so critical if the baffles are angled as the plume reflection will be inward and (due to the flow) upward such that a more elevated area portion of the baffle space can be effective.

(d) I'm unclear what VaH hood is being compared here. Please specify the model number. The VaH Magic Lung hoods with interior squirrel cage blowers may not have uniform flow at the entry aperture. I've never owned one or measured one. I don't think that their open design would naturally induce uniformity. VaH baffled hoods likely behave about the same as their competitors. I don't consider the Wolf 18-inch height to be shallow. Eight-foot ceilings only allow two feet for hood height when mounted 3-ft above the counter-top.

Note that remote blowers allow use of silencers to quiet blade tip turbulence noise, something not possible with hood internal blowers. That is more important to me than minor irregularities in the flow field under the baffles.

I would conclude with the belief that hoods with correct baffle design operating with the same ducting and blower fan curves will operate about the same in capture and containment. If the baffles in one contender are mounted so that the reservoir volume is larger -- at best there is only 6-inches more than Wolf uses to play with in a standard house -- there will be a modest advantage in transient plume capture. But this can be addressed by using enough CFM to handle peak transient plume volumes.

I doubt that designing for 150 CFM/sq. ft. (multiply by 1.5 for nominal blower rating) would fail to deal with grilling activity unless all the meat is on fire.

Now, on the topic of light bars there are potential issues. I changed my halogen lights in my Pro Island hood to LEDs to keep from having to wear a hat underneath. A central light bar in the hood would not have that issue. However, my lights stay clean. An internal light would tend to accumulate grease, I would expect.

As I noted, but I don't have a grill and use induction, I don't get grease accumulating on the light bar or the other edges of the entry aperture, so I conclude that capture is taking place without plume impingement on the edges.

@Steve E I'm glad you mentioned the VAH containment thing, because I happened to see one recently at an open house that clearly had significantly better containment volume than your standard residential hood (ie empty space) and a much narrower light band than most, so it would achieve better capture area given its 27" depth than most 27" hoods. The "squirrel cage" box inside was higher up, not blocking the lower empty volume too much.

I went poking around online after and see this is probably their PRXH18 design. Most of their other hood options are not like this one, typically less deep and less tall.

I know @opaone is quite critical of his past experience with a VAH in the range exhaust FAQ, but from the picture on that blog post it looks like the model they had was not particularly deep or tall? Of course the criticisms of the non-baffle "squirrel cage" blower design still apply. But I'm wondering if models such as the PRXH18 should nonetheless be considered a stronger contender here than it usually is, solely due to its capture area and containment volume. Maybe this could somewhat alleviate the downsides of the blower/baffleless issue, assuming adequate maintenance, ducting, MUA, etc of course?

@d7sharp9 I had the same reaction however there is simple solution to making this an apples to apples comparison. VAH offers the option of purchasing the PRXH18 with baffles for customers who prefer to use an external blower. I've requested a drawing for that model so I can calculate the internal volume compared to one with the squirrel cage blowers. While I expect it to be less, I think it will still be considerably more than models from Wolf and Thermodor.

As you note the volume isn't the only difference between the VAT and the other high end hood. The leading edge is much more shallow as well. If you can make your way through my longwinded post you'll see I'm poking at kaseki in the hope that he will post his thoughts, based on a model for the air flow in each hood, of how these two designs are likely to perform with regards to uniformity of velocity of flow across the hood.

There has been a lot of discussion of what is required for a hood to perform well. Reviewing the literature the answer seems pretty clear. A hood must exhaust enough air so that the velocity of the air flow exceeds the velocity of the plumes coming off the cooking surface. While there can be disagreements over what that number should be you can choose the number you want, within reason, by adjusting the size of the blower.

What you can't set unless you design your hood from scratch is the geometry of the hood. So if hood X which is shallow or flat has a significant fall off for the velocity of the air around the exposed edges (front and all or part of the sides on an undermount, all around on an island hood) then that hood is going to underperform hood Y or which has less fall off.

But by how much? It depends on the amount of fall off but if it falls below the design rate, substantially.

In other words you can't just talk about the flow as an average but need to know the flow at all points of the aperture of the hood. But this isn't all that complicated. Even without a model we know that any fall off is going to be at the unbounded edges the questions is just how much. Building a model isn't difficult. Even so one wonders whether the manufacturers of residential hoods are even aware of the requirements for a good performing hood and if so whether they feels its worthwhile to design theirs the way companies like Greenheck do.

But maybe they are at least aware of the need for a hood to meet a minimum for air velocity including at the unbounded edges that doesn't mean they've designed to meet them. It's likely their primary considerations are price and aesthetics. We just don't know unless someone (us?) decides to buy one and test it.

Now as luck would have it testing them appears not particularly difficult to do. Lawrence Livermore did exactly that and published their methodology along with their test results. But I don't think its a good idea to perform emperical testing without having first modeled to determine the expected results because then one has nothing to compare with which can easily lead to unfortunate outcomes if the testing contains errors which it frequently does.

Which brings me back to dear kaseki. So far he hasn't weighed in on his thoughts on a model for performance that would allow and informed discussion as to how the VAH is likely to compare to a Wolf regarding air flow velocity at the unbound edges.

Moving on to containment I find it much easier to understand the effect of only one variable at a time I make every effort to do so. That is not always possible in a system but I think it is here. One can evaluate the effect containment volume and aperture air velocity separately. This is true even though the former has a strong effect on the latter but why I think the latter should be understood first.

Imagine a fully flat hood with zero containment below the baffles but with near perfectly equal airflow velocity across the entire downward surface so that even the edges are drawing air in at equal to or more than the design cfm. How does that hood perform?

Now slide that hood up and contain the space below the baffles with a box. Choose various heights/volumes and compare the performance. You have your answer.

But meanwhile there is more to be understood regarding how well existing shallow, wide lipped hoods are likely to perform compared to deep, narrow lipped in so far as the face velocity at the edges.

Hopefully kaseki will weigh in on the results of an rough mental model. If he does then we have an expectation for performance making a more formal model the next step followed by a lab test.

@Steve E well if you do get those volume comparisons please do share them!

I've dragged my feet for a couple years now on replacing an Elica range hood. I still can't stand its braindead design (it manages to waste an impressive 7 inches of depth on its lightbar), but even it seems to manage an acceptable venting job ever since we installed a Fantech makeup air system, especially when cooking on back burners which I often do. Kaseki and everyone else here who have long expounded on the importance of MUA were right of course.

But seeing this thread revived recently has reminded me I'd still like to make a move on replacing the hood. I'm not sure I want to introduce the complexity of installing a remote blower any time soon (the MUA system a couple years back was already enough of a retrofit project hah), but if the case for its performance is compelling enough...

I thought I had beaten this to death. Please re-read the thread to try to unify the comments. The issue I have been addressing is not the flow rate at the inside edges, but the flow rate at the outside edges, particularly the light bar. Depending on details, the flow rate at the inside edges will be slightly degraded from the average at the entry aperture due to wall friction, call it 80%, while the flow at the three outside edges without the light bar will be the same for all models and likely about that of the inside edges.

The light bar on my unit is about 5 inches, and the flow at the outer edge, which will be horizontal, is unlikely to be less than 60%.

Also please note that my island hood interior dimension is 26 inches. I don't know what it is for the wall units you are considering, but please draw a side view of your cooking surface vs the hood candidates so we can get a potential capture angle. The plume half-velocity half-angle is likely to be about 6 degrees from vertical. It is this capture question that matters here, not whether the baffles are aligned with the entry aperture or are 6 inches higher up.

To the point of one of your questions: Baffle angle aids grease collection and increases reservoir volume, but this volume is so small relative to commercial hoods that the only difference between my tilted Wolf baffle volume and some horizontal baffle design is a slight advantage for transient plumes. I'm not sure your grill will be a big producer of transient plumes, compared to wok cooking, and without an Asian restaurant 100 kBTUh or greater wok burner, you are unlikely to beat the grill total plume CFM with transients when wok cooking.

Also, in specifying the hood position, the center of the capture area should be aligned with the center of the cooking surfaces, unless this requires wall penetration. This alignment may lead to some gap at the back that a metal plate can address to keep effluent off of the area behind the hood.

Also, where is the grill relative to the center of the range, and are there cabinets at the hood sides?

Another question seems to be missing the point. For the pyramidal hoods such as the Wolf and I expect the VaH, the volume above the baffles is not significant among these contenders. It is the volume in the capture area that has an averaging and transient collection relationship, but again, this is fairly slight unless one has a very large volume there. Here is opaone's hood.

As you may surmise, this reservoir volume so vastly exceeds what you can get from an 18-inch high hood that discussion about the merits of the wall hood designs is essentially an argument about how many microns of gold should be used to gild a lily.

The LL modeling comment essentially asserts that a significant mechanical engineering effort should have been made to construct a computational fluid dynamics model that includes every hood detail, followed by HPC modeling runs. I've always wanted to see such a result, but don't want to pay the taxes needed.

The Schlieren images I provided earlier tell the entire story; either a given cooking activity is fully captured and contained, or it isn't. If you don't want to deal with all of the issues of getting a commercial hood into position over your cooking zone, then you have to live with residential hood designs and available blowers.

On top of this, it would be good if the hood stainless steel has the same color as that of the Wolf range, so that is something to look into.

I'm going to go with a residential hood. The only question is whether its the VAH or the Wolf.

Answers to your questions...

12" deep cabinets abutt both side of the hood.

The burners for a 48" Wolf range are two each on the left and right side with the griddle and grill inboard of those. Both of these come almost even with the front of the front burners.

The 27" deep VAH is several inches deeper internally than the 27" Wolf due to the absence of the wide almost horizontal edge.

The under counter Wolf hood has the same wide front edge design as yours so in light of the amount of smoke the grill would throw off with a substantial portion towards the front of the range a 40% reduction is concerning.

I'll review the thread again as you suggest but in light of the 40% number in your last post it would seem like hoods would perform better if the front edge was more like the edges (which is the case with the VAH).

The 60% flow velocity at the outer edge of the flat section is surmised from a sharp edge hood contour plot. It grows to 100% just inside the inside edge. It also grows as one moves down the vertical direction from the inside and outside edge. The entire front of the stove area is part of the flow path, so determining how much plume might escape requires picturing the flow field vs. plume field all the way from the grill edge upward. When I get a chance, I'll scan the flow field plot I am referencing (for research purposes).

Further, a wall hood has the property of the back wall providing an extension of the hood in that area, along with cabinets. So in a sense the effective capture intake aperture is some sort of curved 3D shape such that there is an effect on the cook's side performance not present in an island hood.

Did you ask Wolf about what combination of blower and the candidate hood would be recommended for your intended stove to assure capture and containment? They do answer their phones and will connect you to engineering in some cases. Maybe they will provide you with a fan curve annotated with a baffle pressure loss curve for the candidate Wolf hood.

Using the Wolf 1500 CFM blower as an example (Broan 336 model), I would expect you could pull a 1000 plus CFM through the hood baffles. (Assumes a pressure loss of 1 inch of water column.) (Expect baffle hiss.) If we assume an effective capture aperture of 8 sq. ft, then the specific flow rate is 125 CFM/sq. ft. or 125 ft/min or 2.1 ft/s or 0.63 m/s. We only have to bias the grill plume a bit (waiting for your angle data) to keep it inside the hood. I would expect that to be achievable with that flow rate. In other words, the flow field under the hood aperture progressively vectorially adds to the plume flow field all the way up to the hood. It is a lot easier to bias the plume flow field on the way up than, for example, divert it 90-degrees to a pop-up down draft vent.

Plan to include the Fantech silencer for your duct size, particularly with the higher flow your grill may require.

Be sure that the hood-mounted control for the blower the hood manufacturer expects to be used works with the blower you choose.

I've spoken to Wolf. They state they know nothing about the hood design and won't provide even basic information such as how to specify the remote blower. I don't fault them for this because they are up front about it. Their expertise is ranges, ovens, cooktops and refrigerator freezers and they make it clear that all design work is up the customer's installer.

The product page for the blowers only has the installation instructions and doesn't include the curves but I seem to remember seeming one somewhere and will look for it.. The duct run is straight up from the hood through the roof a distance of less than 8' with no turns.

I appreciate your willingness to post information regarding the hood since you are the only source of this information. I'm fairly certan that Wolf can't provide performance data on the hood simply because because they've never tested them. Once again I don't blame them for this. It just means one either takes one's chances or you do the work yourself, which is why we are having this conversation.

I need to find out which of the showrooms has a 27" hood to get you the angle data. If none of them do I'll extrapolate from the 24" ones that I know are on display. Give me a few days.

The Wolf hood obviously includes the appropriate controls for the Wolf blower (which is manufactured by Broan).

I've assumed a Fantech silencer.

Thank you for the offer to post the flow field plot. That is exactly the missing piece of information I've been looking for.

Since Wolf once sent me fan curves with hood (which hood undefined) pressure loss lines including various duct lengths, I don't doubt for a second that they once knew the pressure loss vs. flow rate characteristics of their hoods. The hoods were then made by Independent, so they might have forced Independent to provide characterization. If a baffle section were to be tested for pressure loss vs. flow rate, then we could characterize various hoods, ignoring what should be modest transition loss at the duct interface.

As an example, here is a page from FlameGard for a similar baffle filter type. If one scales air flow for the Wolf baffle area to a FlameGard baffle area, one can then read the pressure loss for this similar baffle design. (Click to expand.)

(Some graphical analysis may be required.)

Flow diagram for one hood configuration follows; fair use for research purposes per Title 17 USC § 107. (Click to enlarge.)

Ref: Figure 6, Chapter 30.4, of the ASHRAE Handbook: HVAC Applications.

Note the drop in flow velocity at distances relevant to a cooktop. This is why an overhead hood depends on the plume upward velocity. With more inspection, one might also see one reason why various forms of pop-up down-draft ventilation with narrow entry apertures (W above), have low influence across an entire cooktop for realizable CFM.

How do you think these curves would differ if rather than a plain rectangular opening the profile was as it is along the front edge of the Wolf hood?

I think for the purposes of this discussion, one could draw a straight line parallel to the hood base for a grid block or whatever scales 5 inches, and see that while maybe the relative flow rate at the outer edge is near 50%, under that surface downward towards the counter the flow rate is still effective against the more angular plume rays.

If you print a copy of this figure, and place a counter at 150% (3 ft for a 2 ft hood 'W', define a pan location, map a plume contour (cosine-squared, say, although triangular is closer in some cases to the Finnish measurements) such that 50% of peak velocity is at some angle like 6 degrees from vertical (at the pan edge unless the flame is wider), you may find that the rate of fall-off of hood flow is not worse than the rate of fall-off of plume velocity. In addition, keep in mind that the resulting plume ray direction will be the vector sum with the hood air angles tangent to the gradient lines, so I expect most 'cook's direction' plume rays will be diverted toward the hood entry aperture.