A bit of free association follows. Review the last decade's myriad hood topics in this forum for full coverage. Read the front part of Greenheck's guide [ http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf ] for a basic understanding of the factors involved in kitchen ventilation.

Assuming that there will be a few inches between the duct and the garage ceiling, the garage is an ideal place to mount an in-line silencer to quiet blade turbulence noise from the blower, assuming the blower is outside (or at least beyond the silencer).

Hood front overlap can be commensurate with side overlap, which is really the overlap of the hood internal entry aperture (at the hood bottom) with the largest pan edge (for gas). Depending on how the controls are placed on a cooktop, this may or may not require 3 inches past the cooktop front edge. Wall hood overlap on exposed sides is potentially driven by cross drafts as well as the expanding conical cooking effluent plume, so you have to be the judge of how wide is wide enough.

For required flow rate at full power, aim for 90 CFM/sq. ft. of hood entry aperture (inside area at hood bottom) as an area specific CFM. In other words, total CFM = specific CFM times area.

To achieve the resulting calculated flow rate, the blower has to be sized larger because the blower rated flow rates are determined with the blowers in free air. In practice, the blowers have to overcome pressure losses, both in the path from cooktop to outside, and in the path that the make-up air takes keeping the house pressure at a fixed value, preferably very close to the outside pressure.

With relatively free flowing make-up air, a total CFM factor of 1.5X should cover baffle, hood, and duct pressure losses. A larger factor than 1.5X may be needed to deal with flowing hundreds of CFM through cracks in the wall by depending on "default" MUA. (If there isn't a specific MUA approach, the pressure drop in the house may be sufficient to back-flow combustion appliances, if any, and this is hazardous.) Default MUA will also significantly "gag" the hood blower. Open windows work OK, depending on temperature. Note that window screens present a pressure drop also.

As for hood manufacturers, there are many. You can easily discover Wolf hood configurations by reviewing that part of the www-accessible Wolf Design Guide. Wolf, Abbaka, ModernAire, CaptiveAir, and many others are your (not inexpensive) oyster. There is also Vent-a-Hood, with somewhat different characteristics relative to the residential emulations of commercial hoods that I think are more optimal, particularly where MUA is likely to be supplied at a pressure loss. I happen to have Wolf's largest Pro Island hood, and no other hoods, so I won't try to qualify these manufacturers' relative merits.

Blower actual flow rates versus pressure loss are given by the blower's fan curve. Such curves are available, for example, from Broan's web site. One example, that I have from Wolf for a Broan 801641 blower (900 CFM nominal), shows that low medium and high flow rates for some undefined hood and 40 ft of equivalent duct are, respectively and approximately, 450, 650, and 690 CFM. A second example, that I have from Wolf for a Broan 801642 blower (1200 CFM
nominal) shows that low medium and high flow rates for some undefined
hood and 40 ft of equivalent duct are, respectively and approximately, 250, 450, and 750 CFM. Many blowers are controlled by continuous rotary knobs, and do not have a specific medium flow rate. The lowest flow rate will likely not be zero, but perhaps one-third of the maximum actual flow rate.

Duct size is normally that which the hood and blower are adapted to, but should allow the exhaust air velocity to fall into the 1000 - 2000 ft/min range. Assuming you run the hood for a few minutes before generating a lot of grease, or that the garage air temperature is warm, then the higher end is better. For lower deposition on cold walls, the lower end of the range is better. In either case, assuming decent baffles or clean mesh filters, the grease build up on the duct walls should be minimal when the cooking is typical residential, even over decades.

Kaseki, thank you for the reply. I'm afraid that it's way over my head with technical terminology that I could spent months trying to understand. I guess I just need to know what is the minimum CFM that is adequate for my situation and is MUA necessary for that amount? What is the minimum CFM that I can use without MUA? The only combustion appliance is a propane furnace in the attic. There are open soffits that supply air to the attic. There are also two attic fans that are set to run if temp gets above 100. This is not a sealed attic and is accessible via a pull down step ladder. I am currently looking at the Wolf PWC422418 model. We planned in doing the internal blower. The CFM rates available start at 300 and go up to 1200. The guy at the appliance store recommends 1200 and says that he "thinks I'll be okay" without MUA. Says he's never had a customer that was concerned about it and has never had to deal with it because it is not a requirement of the codes here. I asked him if he would put it in writing that I didn't need to install MUA and of course he would not. My husband thinks we should do the 600 or 900 w/o MUA. I just want to be safe. We do live in the southeastern part of the country and would like to fry green tomatoes and friend catfish sometimes. Thank you for your time.

If we take the intake aperture to be 2 ft by 3.5 ft, then the area is 7 square ft. (It is actually a bit less because these are outer dimensions. But I don't have one to measure.) Seven square feet times 90 CFM/sq. ft. is 630 CFM per my description above. Multiplying by a factor of 1.5 yields 945 CFM for a blower rating.

My possibly dated Wolf Design Guide shows the PWC22418 to have a 900 CFM internal blower option, so I would go with that for internal.

No air will flow out of the hood to the outside without an equal amount of air replacing it by flowing into the house. This replacement air is called make-up air. Either it is supplied via some deliberately open path to the outside (make-up air system), of the hood will flow only as much air as the interior house pressure will allow while the blower is trying to pull air through cracks, open windows, etc. If the house were perfectly sealed, the flow rate of the hood would be zero. If this is too technical, find a HVAC specialist to explain.

So long as you do not lower the pressure in the attic (soffit venting greatly exceeds the area of the cracks around the pull-down ladder, gaps around ceiling lighting, etc.) then you should be safe. I wouldn't run the hood with the ladder down and furnace on. You can (and should anyway) include a CO sensor alarm in your remodeling.

What is safe vs. unsafe is up to the opinion of your Authority Having Jurisdiction, a.k.a. Code Enforcement Officer. I can't tell you what he thinks.

I appreciate your reply. Thank you.