I'll chime in as a Prizer hood/Bluestar owner.
First of all let me say I envy your 48" dual griddle set up!
I have a 36RNB with griddle and if I had room for a 48", I would definitely have a 24" griddle!
I have a remote blower and had the same option of 1000 vs 1400 cfm. I was limited to 8" duct, so went with the 1000, and it has been sufficient although I think given your size and set up I would go with the 1400 to be safe.
I'll add that we live in So Cal where no MUA requirements exist, so I can't comment on that aspect of your situation.

Hey Cookncarpter! Reading through your input (and Mistman's, is he still around?) here on the forum regarding the Bluestar griddle is one the reasons I decided on the double griddle! Thanks for your reply...I think I will go with the 1400 CFM and the 1600 makeup air system.

How do you like your Bluestar cookncarpenter? I have selected a 48" myself (6 burners and griddle), but not settled on a color yet.

eliasgrace:

Your message didn't provide enough information to answer your question from a requirements point of view. OK, your hood is 74 inches long, but how deep (front to back) is it? If you take the aperture area of the bottom opening (L x W)/144 to find the area (square feet) of the opening, and multiply by 90 CFM/sq. ft., you will get the recommended actual flow rate. Multiply by 1.5 for fan curve effects vs. pressure losses (a WAG without actual data), to yield the rated blower flow rate that I would suggest.

Suppose the answer leads to vicinity of 1400 CFM rated. Then the 1200 CFM Fantech MUA system may still be appropriate, depending on which path has the most pressure loss, differences in the blowers' fan curves, and how much house leakage there is at some safe negative pressure resulting from any flow rate mismatch between the hood and MUA air flows. Total MUA flow rate (blown plus leakage) has to equal the actual hood flow rate.

kas

"How do you like your Bluestar cookncarpenter?"
Anyone who has been on this forum for the last 5 years is probably tired of me being a poster boy for Bluestar! haha.
The range is truely a workhorse, no frills, no gimmicks. Out performs anything I've ever owned or cooked on by far. I use the griddle as much or more than the burners, and have never had the need for more than 4 burners. Everything easy to clean and maintain, it is so basic that most anyone could easily do repairs themselves.
Enjoy your new range!

Not at all tired of you pretending to be the poster boy. That just takes the heat off me.

I always feel I must be advertising it too much. But it's such an awesome range, I can't help it. I feel I have to tell the world...

cookncarpenter, is your griddle built in? Since you use it so often, just curious what sort of things you "griddle" vs using pans on burners.

Thanks

Inga

Yes I have the built in griddle. Pretty much anything that can be cooked in a pan
or on a grill, can be cooked on a griddle. Think short order cook at a diner using a flat top griddle.
Also, if an extra burner is a ever needed, a saucepan or two can be put on top of the griddle, and being thermostatically controlled can be used to keep foods warm or simmering for hours...

Only bumping this old thread to confirm the Kaseki math...

Looking at a 36" rangetop with a 36 or 42" hood. I had been assuming 1200 CFM blower was called for but according to the math....42"x24"/144=7sq feet or 36"x24"/144 = 6 sq feet. Times 90CFM = 540-630 CFM blower needed.

I have an opportunity to buy a (used but still new in box) wolf 36" hood with 900CFM remote blower. for $1500...seems like I should jump on it? I could either leave an uncleanable 3" gap on either side and upgrade the hood someday if I find a deal on a 42"....or bring the upper cabinets back in and just live with a 36" hood over a 36" rangetop?

If I understand correctly, you need to add another adjustment factor, as real-life losses reduce the effective flow rate. I think the rule-of-thumb that @kaseki uses is that you would need to multiply your numbers by a factor of 1.5. That would give you a range of 810-945CFM.

Also, frequently people suggest extended the depth of the hood to more than 24". I personally prefer a hood to be larger and to then have it mounted above head level.

That's how you get suggestions of 1000CFM hoods above higher-powered stoves.

I guess even with the 1.5 multiplier...a 900CFM blower counts like it would be really effective if not ideal.

I would like the oversized hood (42"w) but being 6'6" and having 8' ceilings...sticking out more than 24" and/or being above my head are not really options worth considering (for me).

curious with this kind of suction power if anyone uses their range hood for whole home cooling in the summer? Growing up we had an attic fan and would turn that on on a cool summer night, and crack a window in our bedroom and get a cool constant breeze as the hot air was sucked out. Some quick math shows that if you're moving 900CFM and you have a 2200 sq foot house with 8' ceilings you're looking at around 20 minutes to suck all the existing air our of the house and replace with cool evening air if you have some upstairs windows cracked for MUA.

and finally...given the choice...would you go 42"w x 22" deep...or 36" wide x 24" deep? (over a 36" rangetop)

Going by aperture alone, you would get move space with the 42x22...

I really like deeper hoods. But some of that is personal preference. Have you tried building a mock up from card board and seeing which one would actually work in your kitchen, and with your body height.

A mock-up or a good drawing is essential to determine sight lines and head bumps.

Yes, don't forget the factor of 1.5.

42 x 22 vs. 36 x 24 pretty much depends on which burners you will use for the greasiest or most odoriferous cooking, and whether cabinets at the sides help constrain rear burner plume expansion.

Feasible mental computational fluid dynamics can be based on first imagining a set of truncated [10-degree half angle] cones fitted to the burners' maximum likely pan diameters. Then decide how well the most critical expanding cooking plumes are captured by the hood aperture.

Reprise: Capture is dominated by hood entry aperture overlap and only weakly depends of hood flow rate (CFM). Containment is dominated by hood flow rate and only weakly depends on the interior details of the hood aperture in the zone between aperture and baffles. The only effluent that is immediately removed from the kitchen is that which is both captured and contained.

I am a little confused as to how hood aperture can be so relevant.

Say you have a 36" range....its 24" deep. If you have a hood of the same size mounted 36" above it...you have 18 cubic feet of air between the stovetop and the hood. At 900CFM you're removing 15 cubic feet of air per second.

So in the first second your blower is running (you can assume it was already on for a few minutes before the test to get up to speed and have air pressures balance out etc), it removes 83% of the air over the range.

During the second second, it does this again, which means that in that second, it is replacing 15 cubic feet of air under the hood, with 15 cubic feet of air surrounding the hood/range column (that is to say, the column of air between the top of the stove and bottom of rangetop. In many configurations (mine included) the fan is boxed in my cabinets a little bit on either side, and the wall behind it. Which means it only has 18' of surface area exposed, from which to suck the 15 cubic feet of air.

Unless there is some guidance for how to calculate where in that surface area the air will be sucked from first....we could assume it will equally suck in 10" from all sides of the hood/stove column. Assuming a 12" pan on the front burner of the range, and food being fried or burned on the outer edge (aligned with the front edge of the rangetop), the 10 degree half-angle means that effluent rising 36" above the pan, is now 6" from the front of the range/hood...which is within the 10" of air which is being currently sucked into the hood.

I could see where the additional width of a hood in this configuration could be beneficial, but in my mind that is only to prevent effluent from rising from a pan, hitting the bottom of the cabinet and getting it all greasy or being deflected out along the counter away from the hood making ventilation more difficult. I don't necessarily see how with that level of air movement (900CFM), an additional 3" of hood depth really changes the equation. If you had a much lower rate of air movement, then I could see the hood depth being more relevant.

For example, if I had a 400CFM blower, I am only moving ~6.5 feet per minute...which in a hood 2'x3' means I am barely able to pull more than the 12" of air directly underneath my hood. The volume of air being extracted is so low in that scenario, I could see why a larger (deeper) hood would be substantially beneficial, because you dont have enough air movement, to be confident that you will pull out effluent that is in the air, ~35" above the rangetop and ~10" away from the front of the hood/range. It seems like the argument could be made though that with CFM movement that low, you would need an even deeper hood...like an impractical 32" deep hood 36" above the stove.

I guess my unsubstantiated, uninformed hypothesis is that hood aperture is a more important consideration when the volume of air movement is lower (given a constant range surface area)....and that there is limited benefit to a 27" deep hood vs a 24" deep hood if your blower is powerful enough.

So if you had a 36x24 hood mounted 36" over your 36x24 range...and you were trying to improve effectiveness of your ventilation system...it would be better to increase the CFM of your blower than to increase the depth of your hood by a few inches.

Looking forward to your thoughts on this Kaseki and also, can you clarify for me the difference between capture and containment? I googled but only found documents referencing both together (blah blah capture and containment blah blah). Seems to me like capture and containment mean the same thing, and the words that should be use would be capture (or containment) and ventilation or something, where ventilation is the volume of air removed per unit of time.

Argggg. My answer just disappeared.

OK, writing in parts.

Capture:

Capture is the process of getting the rising and expanding cooking plume effluent into the hood aperture. It depends on the hood overlapping the conically expanding cooking plumes. Very little of the expanding plumes that pass the hood outside of its boundaries is pulled in, except over time as part of the room air being removed and replaced. By then, grease, moisture, and odor may have been deposited where they are unwanted.

The air flow into the hood will provide a breeze around the hood, and this breeze may slightly deflect the weaker parts of the plume, aiding capture, but for the majority of the plume, the hood depends on the plume to rise to it.

Containment:

Containment is the process of moving the captured plume content past the baffles where it can be removed (or cleaned in the case of a recirculating hood. For any significant air flow rate, air past the baffles will not be able to move back into the kitchen.

Containment may also be defined as overcoming the plume momentum so that it doesn't deflect or spill out of the hood. To accomplish this, the hood air flow must entrain the effluent into its flow and pull it into the baffle gaps. 90 CFM per square foot of hood aperture will cause a flow in the baffle gaps of 180 ft/s or higher, depending on design. This is commensurate with the highest velocity reported for a hot plume of 1.2 m/s. See column 2, Figure 4 table in the Greenheck Guide for a list of types of cooking for which this value is relevant. In my own situation, induction wok cooking also works without spillage at this hood air velocity. Some commercial cooking conditions, such as a charcoal grill, require higher velocities.

Containment Illustrated:

The images below show failed containment and successful containment, due to having enough air velocity.

On suckage:

"Unless there is some guidance for how to calculate where in that surface area the air will be sucked from first..."

There is guidance, in a sense. If you review Chapter 30 of the 2003 ASHRAE Handbook: HVAC Applications, you will find plots of velocity vs. distance from a hood aperture as a function of the hood's narrow dimension. Flow velocity falls rapidly with distance down and with distance outward. Obviously, the more the hood is surrounded with constraints, the more flow there will be in the area without constraints, and hence the higher flow velocity. The wall behind a wall-mounted hood is one constraint. The side cabinets are a partial constraint mainly relevant to the rear burners. Side skirts, or curtains, used with commercial cooking units can provide some further constraint to the side flow.

Conceptually, one could constrain the system such that the hood aperture was also the constraint aperture, and the flow velocity at the hood aperture was the flow velocity at the cooking aperture. This would yield perfect capture, and with enough velocity, perfect containment. However, the cooktop better depend on induction or electric coil conduction, because the gas flames will be seriously disturbed. Adverse pan cooling may also occur.

Thus, constraints provide improvements in capture. They do not affect containment significantly for hot plumes. That depends overcoming plume reflection momentum.

On drafts:

One way to see the effect of missed capture (without a schlieren photography setup) is to heat cooking oil to the point of vaporization and view its rise. Then using an existing air conditioner or small desk fan, or just by walking around the hood area, observe how easily the plume is diverted away from its normal path. Under such conditions, the air velocity of the hood, weakened greatly by modest distances from the hood aperture, has to dominate the draft. It may not. Hence, a larger hood aperture can make up for the "bent" conical plume trajectory.

A bit more on hood front-to-back depth:

The requirement depends on several factors as one may infer from the discussion above. But the one not yet addressed is what is being cooked on the front burners. A large pan overhanging the heat source might well generate effluent that escapes capture unless the hood overhang is large enough. Weak plumes from large pans (simmering bacon, say), or strong plumes from small pans may not call for a 27 inch hood.

Also relevant is that most residential hoods have controls and lights in the front aperture "edge" and the actual aperture size is reduced by 3 or more inches from the overall hood size. This edge is a special zone not specifically addressed by the ASHRAE plots. Likely the fall off in flow velocity along the surface is not quite as rapid as with a sharp edge, so equivalently, some of that surface may count as hood aperture, but certainly not all for high velocity plumes.