The OP wrote: "Kaseki--So, perhaps the appropriate thing to say is that mesh filters are not as effective as baffles but can work if kept clean? Noise is a concern of mine. I don't want something that sounds like a jet on a runway (exaggerating a bit here). Perhaps baffles are the ideal compromise. " I thought what I said was they can be equal if clean, without explicitly noting that clean is a condition that may be brief. For the same flow rate, I would expect mesh filters to generate a bit more hiss. Fan blade turbulence noise will depend on the fan and other factors, but whatever acoustic level the tip noise is, I would expect little perceived difference to the cook between baffles and mesh filters, everything else being equal. "I know VAH markets itself as much more efficient--e.g. their 300 CFM will actually perform the same as someone else's 450CFM. Are people here in agreement with that statement? I guess if that were true, I might be willing to put up with a bit of extra noise and cleaning hassle. " This has been commented on here in many threads, including a recent one by me. It is unlikely the motors are more efficient (air moved per hour per kw-hr used). Grease collection fraction over the particle spectrum has never been published, to my knowledge, so comparative collection efficiency is unknown. The claim of being equivalent to a 50% greater cfm standard configuration is, in my view, likely to be a comparison of "apples to oranges." Normally, fans are specified as having a particular flow rate at zero static pressure, and as the pressure difference across them increases, the flow decreases. The relationship is typically plotted in a "fan curve." Respectable fan manufacturers will publish the curves for their fans. Broan has fan curves on their web site. For other examples, see Rotron and Fantech (published as tables). What I believe VAH are saying is that in their fan housing, their fan moves X cfm, but if it were in the open as other fans are rated it would move 1.5X cfm. Unfortunately, that is not the whole story. There are other pressure losses in a system, such as duct friction loss, turning loss, transition loss, filter loss, and, sometimes the largest, the lack of adequate make-up air loss (the house pressure goes negative). So, without a fan curve one cannot determine how much lower the cfm will be for a VAH in situ. Not that the situation is all that much better for the fans with fan curves, because the hood and baffle losses have to be guessed at for most hoods (commercial being an exception). And house pressure has to be estimated depending on what MUA system, if any, will be in use. As a gross rule of thumb, I assume that baffle hood systems normally configured without more than 0.03 inches of MUA loss will meet the VAH factor; that is, the actual cfm will be 2/3 the zero static pressure cfm rating of the fan. (Note that more negative house pressure will backdraft some combustion appliances and should be avoided.) kas

It is hard to know where to start here. The flow rate (cfm) is determined by the uprising velocity of the cooking plume effluent and the aperture area of the hood, and not to first order by the BTUs. I would start at 90 times the area in square feet. Baffles will not separate grease from the air at really slow air speeds, so if one intends to go as low as possible, then a mesh that is routinely cleaned is probably better. However, mesh hoods typically have undersized apertures, so capture is degraded at the hood periphery. In other words, the hood is smaller than listed. Baffles will at any speed provide fire blocking, their other purpose. All fans have fan curves, including those made from magic lungs. The fan curve plots flow rate versus pressure drop across the fan, which results from duct friction, duct transition flow disruptions, mesh or baffle restriction, lack of MUA, etc. Typical fan curves are slightly convex, with cfm on the abscissa and pressure on the ordinate. When the pressure drop reaches some maximum, such as an inch or two of water column, the flow reaches zero. At zero pressure drop, the flow is (should be) the rated flow. The pressure drop is never zero in situ. VAH may be counting fan and hood, which can also be the rating used by some others at some times. It depends on whether the rating is for the hood with fan or for the fan only. The VAH rated flow certainly does not include the losses from the ducting and duct transition to the cap at the outside. Unfortunately, unless susceptible to a calibrated measurement, code enforcers will look at the fan rating and not actual flow for enforcing MUA rules. Ideally, they would test for negative house pressure vs. what combustion appliances present are not allowed to exceed without risking back-drafting. The relative loudness and ugliness of outside fans has to be compared to the relative social ugliness of loud inside fans. YMMV. I would not, however, duct to my neighbor's door. Some other path should be adopted. kas

I suspect the 0.4 factor is intended to account for the fan/blower performance in realistic conditions, particularly conditions that correspond to the small amount of MUA that the formula requires. My comments here have usually assumed a factor of 0.67, but that approximation assumes that imperfect MUA won't lower the interior pressure more than 0.1 inch w.c., there is another tenth lost in the baffles, and a commensurate amount in the hood ducting. One-tenth negative pressure, however, is too much for combustion appliances with air intakes that aren't isolated from the kitchen, or for fireplaces. Note that the Wisconsin formula listed by the OP is for cases without such combustion appliances. If there is an intent to filter the MUA duct for anything smaller than a bat, I would at least taper out to a largish filter so the filter pressure loss will be minimal. A smaller sized furnace filter would do, or a window screen if large enough (perhaps two square feet). When arguing for a four-inch duct because it can move X CFM, one has to consider what pressure loss will occur at that CFM. I think six-inch would be better in this case unless the duct length will be very short. kas

First, you should consult your local code inspectors, preferably talking to a real person who can tell you what is required in your jurisdiction. If you decide you must have MUA, it is probably best to hire an HVAC company, who has experience in these kinds of installations. There have been a lot of home-brew DIY type systems discussed, especially over on the Appliance Forum, but whatever you do must be acceptable to the code guys. Don't expect the appliance manufacturer or sales person to give you adequate advice - they will never risk killing a sale by admitting that a MUA system is required which may be more expensive than the range and vent hood combined. The capacity of your vent hood is generally determined by the type of cooking you do, and what you can afford. In my jurisdiction, the code guy looked at what my range's manufacturer recommended for venting. If all you do is simmer, poach, roast, or reheat take-out, the 100-200 cfm recommended by that GreenBuildingAdvisor may be enough. But if you want to stir-fry, sear, or grill indoors, you will need every single cfm you can afford to clear out the smoke and grease from your living space. I really don't think you can grill indoors unless you have a hood that extracts much more than 400 cfm - probably requires 1000 cfm or greater. As for the size of the vent hood relative to your range, the 6" wider recommendation is for optimum performance. But in the real world, many of us have to compromise due to space or budget issues. My particular work-around is to do my high heat searing or stir-frying on a burner that is centred under my hood - at least my cook top affords me that choice, with several burners of the same output. Hope this helps.