While I don't know this for sure, I would bet a few donuts that there is no residential code that requires as much air flow as you will want to have if you are doing serious cooking (defined as cooking with hot surfaces at or near the smoke point of cooking oils and grease).

In my opinion, and this is close to what Greenheck supports for commercial use, an actual air velocity around 90 ft/min at the hood entrance aperture is desirable for the types of cooking one might do at home (with baffle filter hoods). This is equivalent to 90 CFM per square foot of hood aperture. If your aperture were 9 sq. ft., (and the hood was a baffle hood), you would need 9 x 90 = 810 CFM. To get this with a typical centrifugal exhaust fan and the pressure losses of baffles, ducts, and the MUA system trying to replenish this air flow, a larger than 810 CFM blower is needed. Without more information I recommend a factor of 1.5X, bringing the requirement to 1200 CFM.

Now with a VAH, there are no baffles to contribute pressure loss, but there are all the other pressure losses. In addition, it appears from their data that the VAH blowers are less efficient at handling pressure losses. Adding complexity to this guesswork, the VAH entrance aperture velocity for full capture is unknown to me, and may vary with position under the hood.

If I were you, I would use two of their double unit 600 CFM (nominal) blowers. Perhaps this can be viewed as a question of the risk of greater regret -- regret that 900 turns out to be too small, or regret that 1200 was more than needed.

kas

Thanks kaseki. I think we are going to go with the 2 double units. I would rather we have more then needed then not enough.

The most common thinking about the VAH hoods is that they work very well, but are significantly more work to clean than a hood with baffle filters. Since you are getting double blowers, I would ask the salesperson to show you how they come apart in order to clean them. There are the housing pieces, and the two fans, and the surface area which is not covered by filters. Ask the salesperson if you can put them back yourself to see how it's done.

Make sure your exhaust duct diameter is 10" for that amount of cfms.

A full shield AND blower housing clean on a VAH is truly a PITA but does not have to be done very often for most cooking styles. Occasional cleaning of the blower shield is very easy. I used to only do the full-boat ( & wheels) cleaning and curse the process every time until I learned that was not necessary - just clean the shield. The hood itself is a great performer (I have a 36"/600CFM over my BS range top). By right you should be looking at at least a 3" left and right overhang - so a 54" VAH. I should have gone with a 42 but live and learn. Here's the cleaning instructions, a bit downplayed if you ask me. Keep in mind that you are reaching up and over your range and stretching awkwardly to get at these components. http://www.ventahood.com/index.php?option=com_content&view=article&id=386&Itemid=400