We have the Viking 1200cfm hood (with external fan motor). ours is installed with the 10" 90 degree elbow mounted right to the hood. The duct work goes through the exterior wall inside the 12" high cabinet that is above the hood. I my opinion it works just fine and the noise level is rather low. Even on the highest setting the noise is still no more than the fan on our old over the range microwave fan (though this could be due to the external motor). For what it is worth ours was installed by Viking factory trained installers and is covered under warranty. I never noticed any stipulation about the 24" height requirement. Since I purchased all the appliances and coordinated the installation I read all of the manuals and never saw that.

IÂll share what I am doing about makeup air. I need to qualify this with the fact that we have not closed the house yet so performance is still to be determined. I researched commercial installations. If you are obsessive I recommend http://energy.ca.gov/reports/2003-06-13_500-03-034F.PDF. A recommended setup is to replace about 80% of the exhaust air with makeup air at the cooktop. Pulling the rest of the makeup air from the room seems to improve capture. We have a sealed combustion oil fired boiler and no fireplace so I am not too worried a slight negative pressure. Even with new construction there is still some infiltration. For example, the baths and laundry are vented to outside. Another thing to consider is the temperature of the makeup air. In cold climates it may be desirable to temper it. I am going to try it with no tempering, but I have made prevision to add a duct heater if it is required. My plan was to pull the makeup air from the outside; however, my builder suggested we pull it from the unheated attic. He thinks that increasing the air exchanges in that space would be a good thing, IÂm not sure. There are lots of ways to deliver the makeup air. One of the better ones is to deliver the air under the cooktop. My cooktop will be mounted in such a way to create a 6" x 40" channel under it. The end of channel is covered with a decorative grate. The air flows under and to some extent through the cook top, around the front, then up and out the hood. This pattern creates very effective containment of the plume.

I needed more than 600 cfm, and the next size they offered was 1200 cfm (two 600's working together). That's internal blowers venting straight out the back, so there isn't really a drop off in amount of air moved like one would get for extended ductwork. There's a continuous speed control knob. I don't know what the lowest volume is, but I'd guess, comparing my previous hood, that it's in the range of 450-650 cfm. If I'm just stirring up a simple sauce, sauteing an onion or boiling up steamy stuff, I run my hood pretty low. Partially that's for the noise, but really it's all that's needed. If there isn't any steam or smell going around the house, that's all I care about. For frying, boiling down stock, or cookingpalooza for the whole gang, I run it at medium high to high. Medium high is usually enough unless I have all burners going. If there's a correlation to how far I turn the knob it's probably around 900-1000 cfm. I don't have a make-up air system. The experts didn't think it was necessary given the particulars of my house. There's little opportunity for backdrafting, though I have sometimes wondered about the pressure thing. When I have run it at full power, I have sometimes felt that the oxygen was being sucked out, but that might well be part neurosis and part from having dozens of people inside with the doors and windows shut and damp out (which is why the shut doors and windows). At that point, the gas burners and the ovens are heating the kitchen anyway, so I'll just open the little casements in my garden window (like wing windows on an old sedan). There isn't a lot of in-sucking at the windows, however, so I think it's more the people breathing the air rather than a real problem with vacuum/pressure. The house is insulated but not air tight. Hope this helps.

I think it really depends. Nothing that moves that much air is going to be quiet. If you can have remote blowers and inline mufflers, then you can reduce some of the sound; and in that case, a VAH might very well the obviously inferior choice. But even then, you still have the noise at the baffles. So, don't expect the hood to be whisper quiet. We have a 20 odd year old VAH. As long as the vent pipe is adequately sized, it works great and noise level is IMHO in the expected range. We also have a new ModernAire hood with internal blowers. It has more power than our old VAH, but it can be really really loud. And part of that is simply the whistling sound that the air makes when it passes the baffles at full air velocity. If I turn down fan speed, it's about as loud as the old VAH. In other words, I don't really perceive a big difference between either designs. Pick what works best considering all your other design constraints. And absolutely make sure that your vent pipe is as large as you can make it.