You need more than 600 CFM!

"I know that I should probably have at least 600 CFM and baffles. I would
like LEDs, but I suppose halogen would be fine. I also understand that
size matters, but that 600 CFM should work for something that is less
than 6 sq ft. in area (3'X2')."

At 90 CFM/sq. ft. times 6 sq. ft, 540 CFM of actual flow will be sufficient, maybe a bit less for slightly smaller aperture hoods. However, this does not mean that one can use a 540 or 600 CFM rated blower. The blower ratings are made with the blower effectively hanging in free air (zero-static-pressure flow rate). In a hood, with pressure drops across baffles, ducting, and pulling that many CFM of MUA through the cracks in the walls, the blower rating will have to be much higher -- perhaps 900 CFM if the cracks are large.

If combustion appliances are in use, there are safety issues that need to be addressed. They either need their own MUA, or the house should be tested for whether the internal pressure is too low to avoid back-drafting the appliances when the insert is running full speed . Gas appliances can impose a risk at -0.03 inches w.c.; blown oil burners at -0.06, if I recall correctly. (There are tables for this out there on the WWW.)

Wow- Thank you! That was my calculation too. I had been thinking that I could get away without, but I think I probably need MUA of some sort after all, even with a less powerful hood. Right now, there are no other backdraft issues, but it still seems to make sense anyway. (The natural gas heater draws air directly from outside for combustion and exhaust.) One reason I was thinking about Kobe was that they say they are tested for CFM as installed (I don't know whether to believe them or how to double check this):

"Some appliance manufacturers quote misleading CFM Blower Ratings, not bothering to adjust for the CFM Performance drop once that blower is installed in their hood body and exhaust air is forced through baffle filters and out the vent pipe. An inflated 1000 CFM Blower Rating can easily translate to less than 800 CFM Performance Rating. All Kobe hoods are tested to industry standards under real-world installed conditions for a true performance rating. "

Now I just need to figure out how I can get MUA in there...

Maybe I can use a MUA damper and connect it to my heater return. Many seem to work only with their brand of hood. Are there any that work with any hood?

@kaseki, I'm confused about your 'pressure loss curve'. Sounds the same as a fan curve?

For any system you should be able to add up the pressure losses of all of the components (hood, straight duct, 45, 90, silencer, outlet, etc.) to arrive at the static pressure for the system and then that point, 6.4" SP for instance, on the fan curve will tell you the CFM that the system will produce.

Forgive me for probably adding to the confusion, but isn't a 10" duct needed at 600+ CFM? Perhaps I'm wrong?

opaone

The fan curve on a graphical chart of flow rate (abscissa) vs. pressure loss (ordinate) has the rough shape of an upper-right section of an ellipse. This is a plot of potential flow vs. potential pressure loss and is normally unique to a particular blower (fan + housing, where present). It may be perceived to be similar to a plot of battery voltage (pressure) vs. current drawn from the battery (flow rate).

The actual pressure losses through ducts and baffles, etc., are a function of the flow rate! So one cannot just add them up because the result may fall on the fan curve plot somewhere that the fan can operate at. The actual pressure losses vs. flow rate plot as a line that starts at {0, 0} on the fan curve plot and extends right and upward with positive slope, intersecting the fan curve at some {flow rate, pressure loss} value. The point of intersection is the actual flow rate that particular combination of blower, ducting, and MUA path resistance. While the HVAC parts are usually too poorly characterized to trust the line (estimate) very far, such a line if provided for an actual hood and some baseline duct length and diameter can provide a basis for evaluating changes in duct length, etc.

2ManyDiversions

Duct diameter should attempt to keep the flow velocity in the 1000 to 2000 ft./min. range (lower number is better for cold ducts). As one might deduce, this requirement is also recursive with respect to the plot description above, so one might want to plot a pressure loss line for each candidate duct size. For most modest residence configurations and reasonable duct diameters and MUA paths, the baffle and MUA pressure loss will dominant duct pressure loss. If one's staff's kitchen is in the basement and one's mansion is four stories high, then duct loss may begin to dominate hood and MUA pressure losses.