Yes, absolutely. The more difficult route to vent, the stronger the fan needs to be. More bends equal more restrictions and less effective venting that can only be could teracted by a stronger fan or less bends.

The issue is that fans and blowers are specified by the amount of air they can move per unit time at zero static pressure -- in effect the air moved when just hanging in the air. As the difference in pressure between one side and the other increases, the fans move less air per unit time. A graphical plot of this is called the fan curve. What one wants to evaluate (but rarely can precisely due to lack of information) is what the pressure difference through a hood and up a duct will be as a function of flow rate, and compare this with the fan curve to see what flow rate will actually happen.

You will see a pressure drop moving air through a duct, and it will be larger per unit length at a bend. Also, there is a pressure drop at every shape transition, such as within the hood. Plus, the hood's filter -- baffles or mesh -- causes a pressure drop in proportion to the flow rate through it. But wait, there is more, if the house isn't open or have perfect make-up air supply it will have a negative pressure further degrading flow.

If you want to be conservative, and have no other information, multiply the desired flow rate by 1.5 and buy a fan that is so rated at zero static pressure.

A desired flow rate of 90 CFM per square foot of hood aperture should also prove conservative for most projects.

kas

Thank you everyone!

A belated follow up! I would love an opinion on this hood:

I am considering a Zephyr Venezia 42" wide, 24" deep chimney style hood. 715 CFM. I have 8 feet of ducting, with one 90 degree bend. I am getting a 36" Verona range. Here are the burner outputs:
12,000
12,000
16,000
6,000
6,000

Thank you!

This hood has aperture area well under the 7 square feet one might initially hope for from the expressed dimensions. The capture area does not overlap the cooktop as much as one might wish. On the other hand, with maybe (guess) only four square feet of actual aperture and surely (guess) more than 400 cfm in situ, the air velocity (more than 100 ft/m) at the meshes should be enough to ensure capture and containment of the effluent that rises to intersect the meshes. Effluent initially impinging on the non-mesh surfaces may be partially collected, or may escape.

Bottom line: Probably adequate, but adequacy will depend on the anti-grease / anti-odor fanaticism of the owner.

Be sure to keep the mesh filters clean. Effectiveness of mesh filters, and particularly the air flow rate, drops with dust and grease collection, unlike baffle designs.

kas

Thank you for that feedback, kaseki. Should I consider increasing the CFMs? Or - do you ever keep the same CFMs but decrease the hood width? (still trying to understand the mechanics of this....)

Ideally, you start with enough aperture to capture the rising expanding effluent. Then, having the aperture defined, calculate the CFM necessary to achieve a velocity equal to the rising effluent velocity. This would require 180 CFM per square foot of aperture for 3 ft/s rising effluent if there weren't other considerations. One of those is that with baffles, that velocity only needs to be achieved over about half the area due to the aero shape of the baffles and the flow around them. I didn't figure this out from computational fluid dynamics, but from the behavior of my Wolf Pro Island hood dealing with my induction wok which is offset towards one side (the induction cooktop is offset toward the other side). I expect mesh filters to behave similarly due to the mesh blocking at least half the area of the filter.

While a fairly poor analog, if one thinks of the effluent as billiard balls in a zero gravity field hitting the hood filter, one can appreciate that conservation of momentum will cause those hitting a flat surface to bounce away from it such that the angle of exit equals the mirror image of the angle of arrival relative to the perpendicular to the surface that the effluent hits (just like light). The need is to change the momentum of the reflection by entraining the effluent into the hood air flow. Ideally, this is done as the effluent reaches the baffles/mesh and there is little bouncing reflection going on.

You may appreciate that the flat surfaces of your nominal hood where there is no mesh will be regions where there will be effluent reflection. Some of that will be captured by the air flow which pulls in from a range of angles.

In any case, for best operation, one wants the actual hood aperture to encompass all of the rising effluent and not make the aperture smaller just to get the local air velocity up. (A commercial kitchen hood is an example what is relatively optimal for maximizing capture while minimizing the cost of air handling.) But the bigger the hood, the more CFM needed, and the more make-up air CFM that has to replace the air in the kitchen. This not only adds the modest cost of ducting in MUA, but significant cost if the MUA has to be pushed by a fan and the fan has to be controlled to keep the house pressure in a narrow range. This can be more complex in some ways than large commercial buildings which can run MUA continuously and use pressure differential across a damper to control whether the fan output just circulates around in the fan loop or is pushed into the building.

I think my previous post suggested that for the hood aperture you have, the actual CFM achieved will probably be enough. But imperfect capture and containment should expected from the hood aperture design due to the flat areas not being capture areas.

kas

Thanks again for the information kas! I need to read it a couple more times to fully absorb everything. :)

Hi Rebecca3142, i'm a little late to this party but i can assure you from all posts by kas proper exhaust hood sizing, exhaust ducting, makeup air provisions need to be considered in great detail well in advance of buying/installing pro-style ranges/rangetops AND hoods. We have a 36" rangetop with infrared grill (Monogram). We have a 36" 1200cfm Kobe hood installed on outside wall with approx. 2 feet of 8" galv round pipe. Backdraft damper installed within this pipe. This prevents cold drafts coming down the exhaust when not in use. In addition to the outdoor wall vent cap with 'flapper' style cover. The rangetop has 4 x 18,000 btu burners and the grill is 14,000btu. Therefore total range output is over 80,000btu. Almost as much heat output as our furnace. We regret not going with a larger dimension hood (wider AND deeper) when we use the grill in particular (rarely) with the hood on 'high' we have to open at least 1 window (2.5' wide by 3.5' tall window) to get enough 'Makeup air' (MUA acronym in kas post above) to vent the smoke created from whole cuts of meat, kebabs etc.
We were told to allow a Minimum (very low in my opinion) of 200cfm of exhaust for every linear foot of range width. As you can see we have 400cfm for each linear foot of range and it's barely adequate when grilling. However, you may not be expecting to 'grill' indoors and therefore require less exhaust power. At the very least though i suggest you ensure the underside/opening of hood dimensions are the same size or larger than the stove/cooktop itself. Specifically the depth as this seems to make a big difference in 'capturing' rising smoke/grease when frying, sauteing and grilling. The sales rep where we bought our appliances mentioned this but made no mention of makeup air. Just as crucial if not more important IMO. In addition none of the literature from either GE or Kobe mentions how to provision for makeup air. I guess they assume you know this stuff in advance...go figure.
sean

Thanks Sean! That is very helpful. I haven't order the hood yet, although my HVAC guys are needing a decision (proabaly for the rough-in?).

I selected that Zephyr hood specifically because it has 24" depth. So many hoods I looked at were not as deep. But the the CFMs are on the small side (715) and we do have a bend in the ducting...so I am not as confident on that part of the decision. We are not getting a super powerful pro-style stove, and we don't fry or grill indoors, so maybe it will be ok? I would love to save money, but I don't want to regret the decision.