I would say if you are into "daily Asian style cooking" better to be safe, and go with the 1200.
I have a 1000 cfm remote, and it just barely deals with the smoke from all the wok and cast iron searing I do...

Even 1200 is marginal for woking. I used the wok last night, turned the fan on for about 5 minutes before I started, only cooked for about 10 minutes. Later than evening my DW mentioned how you could still smell the cooking smells even though the fan was on high. I told her to imagine what it would have been like w/out the fan. I only use the wok about once a week, so maybe twice a week I'll use a hi-heat setting (searing also), 1200 works fine for us with a 36" BS RNB.

BTW- I have a BS (Pro-Line) 42" 1200 cfm pyramid hood.

Given adequate capture aperture, effectiveness is based on having an air velocity at the baffles that is commensurate with the uprising smoke and water vapor velocity, about 3 ft/s for hot cooking. This requires at least 90 CFM/sq. foot over the aperture so that the baffle gaps see around 180 CFM/sq. foot. I find this adequate for wok cooking on my induction wok unit. I can imagine possible higher needs, particularly from 100k BTU/burner commercial wok ranges, but for induction I think the 90 value will suffice. Higher will pull smoke in from the edges of the hood a bit more, but it is a small bit.

As to the question of what size blower to use, we need to consider a lot of things, but lets start with the 2 x 2.5 = 5 square foot aperture, and multiply by 90 to get 450 actual CFM. The baffle loss at these flow rates may be 0.1 in. w.c., and there will be other losses in hood transitions, duct friction, and imperfect make-up air (MUA) supply. The blower flow rate drops with pressure loss around the loop from outside to outside. These losses can require the rated capability of the blower to be around 1.5X the requirement, or sometimes even higher. So we start with the derivative assumption that we will need a blower rated for 450 x 1.5 = 675 cfm. For this case I would pick a nominal 800 CFM rated blower, if a compatible one exists for hood installation, in-line mounting, or roof mounting depending on other design and compatibility factors.

A 1200 CFM rated unit will also work (given a reasonable fan curve), and provide more margin. Where larger than necessary helps is not particularly in effectiveness, but in lower noise from blade tip turbulence than a smaller fan running at higher speed to move the same amount of air.

A 1200 should have a 10-inch duct, and if roof mounted with an in-line silencer, (which is about 14 inches in diameter) careful consideration of duct routing may be needed. An 800 may get away with an 8-inch duct and the silencer will be commensurately smaller. Ideally, duct size should provide a duct area such that the velocity of the air and effluent at the actual flow rate is in the range of 1000 to 2000 feet per minute. This minimizes grease precipitation on the duct, depending on duct temperature. I'm not going to calculate all of the flow rate vs. duct diameter options here, but it is straightforward to do.

The big gap in this plan is whether the hood aperture sufficiently overlaps the wok that one is cooking in. If the wok is used on an edge hob, (which I think is all you get with 30-inch induction cooktops) then a 30-inch wide hood will not capture some of the rising effluent, which spreads out as it rises.

Better would be a 36 inch hood, which in turn requires 540 CFM actual, or nearly 810 nominal. In my experience, one needs at least 10 degrees of angle between wok cooking zone edge and hood overlap, both to the side, back (on an island), and front. So when this is not easily achieved, then more CFM will help clear the room sooner, and may narrow the rising effluent angle a bit, depending on counter configuration, but will not allow full capture making up for too small of a hood aperture.

kas

And lack of free flowing MUA will certainly restrict the actual CFM well below the rated CFM.

Wolf provides some Broan-NuTone data for the 801642 1200 cfm external blower. I don't recall what hood that blower was recommended for, but the fan curve shows that with the hood and 10 ft of duct (size not specified), the actual flow is a bit over 800 CFM. At that flow (and likely with an assumption of perfect MUA), the hood and baffles alone were responsible for 0.7 inches of water static pressure loss.

Bad MUA could cause a further pressure loss of up to an inch of water column.

kas

For those who were wondering about the terminology of inches, w.c., note that if a vertical tube with a vacuum inside is opened to water below it, the water will rise until its weight in the tubing balances atmospheric pressure pressing on the water outside the tube (about 406 inches). An inch of water column is a small fraction of an atmosphere (about 0.0025 atm). US HVAC widely uses inches, although transition to pascals is in progress.

Kas, just so I did not misunderstand you. A 36" wide would be better with a minimum of 540 cfm but I think they only come in 1200 cfm.

Just my luck. The local dealer has one in stock and wanted to get rid of the 30" for a reduced price.

As for the duct, I presently have an under cabinet useless hood. It is installed against an outside wall. So even though it is a fairly short run of duct work, I understand that the 90 degree turn adds to the duct run. I will be asking the GC to increase the size of the opening.

From what I have read, the bigger the opening the better correct? Possibly a 10". Regardless of the manufacturer recommended opening size? I don't know much about HVAC and am assuming that the HVAC guys will be able to fit the ducts accordingly.

For wok cooking in particular, you want the hood to extend over the rising and expanding effluent as widely as practicable.

Minimum maximum air flow rate should follow the suggestions above.

Duct diameter in general should be determined by staying between the air velocity limits I described above, but for a straight pass through a wall or even a short 90-degree section this is really of little consequence. Matching the diameter of the hole in the hood is probably sufficient if the blower is in the hood. The hole is probably an appropriate size. In any case, the installation guide will provide recommendations.

Download and read all available literature pertaining to what you want to buy. That way the only surprises will be what they forgot to mention.

kas

I too like wok cooking, since it's fairly warm most of the year here, my solution is to put mine outside. I've got a 60k BTU Firemagic power burner I'm installing, it will have a hood and vent (for the adjacent gas grill as well) to help disburse the effluent since the prevailing winds may trap some against the backsplash, but having it outside seems like it will eliminate some of the potential headaches.

"Kas, just so I did not misunderstand you. A 36" wide would be better with a minimum of 540 cfm but I think they only come in 1200 cfm."

Just so there is no misunderstanding, 540 CFM is my suggestion for the minimum actual flow rate you should have available on full power, which REQUIRES a larger than 540-CFM-rated blower. A blower rated at 1200 CFM is fine if it is available for the unit of interest, and you have a means of running it at lower speeds.

kas

Thanks Kas. I did get that part about 540 cfm being the minimum. Also from reading your many other posts on the fact that running a larger unit on low is better than running a smaller unit on high. The 600cfm would have to operate on high daily.

Thank you so much for clarifying things for simple folks like me :)

This is a difficult area to be definitive about because there are many compromises that have to be taken by the blower manufacturer and the hood manufacturer (if different), usually the needed blower and hood data are not available for analysis, and the characteristics of the house that the ventilation parts are to be used in are not known unless a leakage test is performed and cross drafts are considered. So a lot of guessing is inevitable, no matter how much time one takes to become informed on the topic.

We all (and in particular I) want to have a value in hand to help select from among candidate options, but in circumstances of sufficient complexity that full analysis is impractical, at best one is reduced to some rule of thumb that may safely extrapolate from a successful installation, or it may prove to be carried too far from its range of validity.

Obviously, in the case of Mistman's configuration, the 1200-cfm rated blower is barely adequate for wok cooking, or inadequate if one is intolerant of left-over odor. Without knowing every particular of his configuration, it is not possible to make an informed guess why capture and containment are not being fully achieved. One can at best enumerate the various possibilities.

For anyone starting from scratch, and contemplating the six-month or more climb up the learning and planning cliff of a kitchen renovation, I can only suggest for ventilation that the various sources of information on line, mostly commercially oriented, be reviewed to gain perspective about how cooking effluent forms a plume that rises and intersects a hood and then ... -- well it depends.

The published Schieren photography for enough or not enough air or overhang may provide insight. Using the google search terms "schlieren photography kitchen California," for example, leads to, among other results,
http://www.fishnick.com/ventilation/ventilationlab/.

Note that the image at the link refers to 165 vs 220 cfm perlinear foot (of hood edge excluding the back but I think including the sides). This is for standard configuration commercial cooktops and vent hoods, and you can be sure that the value is actual cfm, not blower-rated cfm. And for these types of tests the MUA is carefully infiltrated into the test kitchen in a way that minimizes drafts.

CFM per linear foot is one rule of thumb. More refinement can be managed by accounting for the type of cooking (see the "Finnish" papers referenced at my Clippings). Greenheck in their guide relate their own rules of thumb.

kas