I think you will be fine. The capture area is important along with CFM. Technically you're reducing your negative pressure somewhat but the capture area is better.

In principle, and pretty much in practice depending on hood shape below the baffles (shape and depth mainly applicable to deep commercial hoods) it is the air velocity at the baffles that must remain constant, and hence the CFM must increase with baffle area for the same effectiveness against the hottest cooking (that which for gas cooking causes the plume velocity to exceed a meter per second), and for induction cooking over 0.6 m/s.

The reason that the velocity must be met over the entire baffle space, even over those parts not used at a given instance, is that we can't conveniently block off the unused parts. In any case the plume spreads out, albeit with lower velocities at the greater angles. I suspect in any case that providing an array of baffle assemblies augmented with adjustable louvers just to minimize air flow requirements would not be welcomed by most.

Aim for 90 ft/s actual average flow and multiply by the baffle area or intake aperture area in square feet to get desired flow rate (CFM). Multiply by 1.5 to get a rough compensation for pressure losses in selecting the zero static pressure rated flow rate of the blower. (With a tight house and no MUA, this factor may not be enough.)

Underwhelming velocity at the baffles leads to this:

kas

Kas I cannot tell what "this" is but if I understand the rest they have about 10' feet under the hood and so need at least 90x20=900 cfm so with 1200 they should be fine- correct?

"This" is a commercial hood over a commercial griddle imaged by schlieren photography showing the heated air turbulence cells rising to the hood, and in the case of some cells, escaping out of the hood. Load the air with grease particles and they too flow out of the hood. (Image from CKV_Design_Guide_2_031504.pdf.) What one wants is the result below, in this case achieved at 220 actual CFM/linear foot of hood, equivalent to 1100 actual CFM for a five-foot hood. The previous image was for 165 actual CFM/lf, equivalent to 825 actual CFM for a five-foot hood. [Linear foot scaling is based on a different but well supported in practice rule of thumb for standardized commercial hoods.]

In the OP's case, 900 CFM is the desired flow rate to meet the desired air velocity over the aperture. A 900 CFM rated blower will only move that much air per unit time hanging in the open, that is, at zero static pressure across the blower. That is the standard for how blowers are rated. Reality intrudes. When flow is restricted by baffles, hood transitions, ducts, duct caps, and imperfect make-up air, all blowers flow less than their zero-static-pressure rating. The amount less is provided by their fan curves vs. estimated pressure losses at various flow rates. A fan curve for the OP's blower may or may not be available. Pressure loss effects are the reason for my suggestion of a factor of 1.5, which will take the 900 CFM result to 1350 CFM as a desirable rating.

There is a lot of slop in these measures, and a 1200 CFM rated blower may well be sufficient for many, and 1500 CFM necessary for others. This will be due to different ventilation configurations and different nose sensitivities.

We need to know the OP's MUA plan. From it we might be able to estimate whether more or less than 1350 rated CFM is appropriate.

kas

Got it. Thanks.

Kas to the rescue yet again! My DH is dead set again a remote 1400 cfm so keeping fingers crossed that the 1200 will do the trick. Sheepishly have to admit that we don't have a MUA plan YET. Was trying to nail down the hood first. Kas, do you recommend we start by looking into the Fantech system?

(As an aside, external blowers allow lower noise systems.)

There are twp residential approaches to MUA. The first is for residences that do not have combustion appliances using household air. In this case the MUA need only supply air at a sufficiently minimal pressure drop to keep the hood ventilation capability where you want it. 0.1 inch w.c. of pressure drop is OK in that case (typically) and a passive system (only a damper and screening) is adequate. This is equivalent to an "automatic-opening window" with a screen. I think Broan has a controlled damper for this, and for really big systems, Honeywell can be looked at.

This approach also includes deliberately adding isolated MUA capability to the combustion appliances that are present so that they effectively are not impacted by household air pressure.

The second approach is for residences that do have combustion appliances that use household air and there is no expectation that they will be configured to have their own MUA. In this case the MUA must not allow the pressure drop to fall below 0.03 inches w.c. relative to the outside (0.06-ish if the only combustion appliance is a blower driven oil furnace.)

In this case active MUA is needed; that is, one with its own fan and a control system that keeps the house pressure up where it is needed. These can be very complicated or merely complex. Fantech has a line that combines necessary pieces that should be adequate for most.

The Code Enforcement Officer has the final say on what is acceptable. My comments above are related to safety and vent performance and not to whatever some HVAC-challenged politician (tautology warning) wrote into a state code. Your local code enforcement office should be consulted on requirements unless you trust your contractor to be all-knowing.

kas

Kas, we have gas heating so it's safe to say that we will definitely need active MUA. Hoping it will be merely complex rather than very complicated! Gonna look into Fantech this weekend.

As an aside, I remember you saying that you vented your wall ovens? Excuse my ignorance, but how do you go about installing a vent for wall ovens? Is there some sort of hidden wall hood insert? It would be wonderful to have something like that to suck up all those oven vapors. I'm assuming this would make MUA much more complicated though...

Unfortunately, most wall ovens do not have their own vent interfaces. What I did was put a pair of registers in the ceiling over the ovens, with their connecting ducts joined in a Y-adapter, and the output duct connected to a roof blower. The blower was a re-purposed NuTone down-blast roof blower purchased many years earlier to provide a boost to the vent of a Litton "kitchen center." The register boxes take (IIRC) 12 x 14 furnace filters to help keep grease particles from the ducts.

Note that not even a commercial "eye-brow" hood will capture the smoke escaping a broiling oven when the door is opened. What these registers do is help pull effluent from the ceiling area and slowly clear the kitchen in concert with the main hood.

The only way to successfully deal with opened ovens is a large hood right over them, or a commercial porous ceiling ventilation system, such as illustrated here:

http://www.kitchen-ventilation.co.uk/heydal

Yes, the MUA now becomes more complex because the control has to be "closed-loop" instead of "open-loop" and thus needs a servo control system to control an even larger MUA blower. My kitchen blowers' combined output is 2500 CFM (zero static pressure rating), and likely pull a total of 1500 CFM when there is sufficient MUA. With both blowers having variable control, the actual CFM can be anything, windows can be open or not, bathroom fans can run or not, etc., so the scheme is to control on differential pressure between my kitchen and the vented attic.

Not all parts of this system have been purchased and integrated. My excuse is that my honey-do list is very long and keeps expanding.

Items in place:

• Roof down-blast housing on pedestal for air intake interface with attic

• Hydronic heat exchanger in attic with its separate control loop to my furnace (functioning)

• 3-ft square diffuser in hallway facing kitchen

• Sensing system installed

• Control system components mounted in basement; Fuji computer purchased but not yet incorporated

Key elements yet to be obtained (mostly from Fantech, I expect)

• Big axial blower sufficient to flow around 1500 CFM through the pressure losses of the ducting, air filter, and heat exchanger. It also has to fit through the attic access stairway.

• Current-loop control fan motor driver

• Pleated air filter box

• Damper

• Various sheet metal interface sections

I should add that my only combustion appliance is a blown oil burner that has its own sealed MUA supply, so delay in completing this kitchen MUA system is an annoyance, but not a hazard.

In your case, a carbon monoxide warning sensor is suggested.

kas

Wow, Kas, your home is going to be the most ventilated place on earth! Reading through your last post, venting my ovens is way too complicated for us...and we can't do the additional make up air. It would be amazing if we could since I like to cook a lot of smoky things in the oven. Oh well!

The lower half of my Wolf dual wall ovens evidently failed during self clean yesterday, neither element heats, so presently I can do only half smoky cooking. :(

Ouch, sorry about your Wolf! Hope it gets resolved quickly!!

Hello Kas:

We're planning to install a range hood from MA with a 1200 CFM.

Searching for a suitable roof cap with proven performance to support 1200 CFM seems more daunting than I've expected. So, I would like to hear your recommendation of roof cap models that will definitely work with a MA range hood with 1200 CFM spec.

Thank you very much in advance for your time.

Best regards,

Wokcooker

I don't know much about roof caps, as my big blowers are on the roof and the roof cap is an integral part of them. I've never had to research roof caps, other than the down-blast housing I use for an MUA air intake. The caps with integral blowers, such as Broan/Wolf and Abbaka make, have their own dampers. This type blows down the roof slope. (It is possible that Broan sells "unpopulated" roof blower housings for use as a cap.)

In locales with deep snow, commercial pedestals to interface the roof with a higher cap may be desirable. MA may be able to suggest appropriate cap sources.

If at all practicable, however, I would recommend roof mounted blowers as this moves a large noise source away from the cook, and opens the possibility of using a silencer (e.g., from Fantech) to remove the blade tip turbulence noise, leaving mainly baffle turbulence noise.

kas

Hello Kas,

Thank you very much for the feedback.

Wokcooker