Think of the effluent rising from a pan in a conical shape. The angle of the side of the cone is about 22.5 degrees, and thus the overall cone expands at about 45 degrees. The effluent that is intercepted by the hood (and doesn't curl back out of the hood) is exhausted, the rest distributes itself throughout the house and is exhausted mainly by air replacements. This may take a long time and waste a lot of heated air. Hence, at least for burners on which one cooks smelly or greasy foods, the hood has to overhang the conically expanding effluent. Generally, one wants the air flow at the interception of the effluent with the hood to have a velocity at least as great as the rising effluent (about 3 ft/sec = 180 ft/min). (Otherwise it may escape the hood.) This velocity is lower than in the duct, but should cover the entire entrance area of the hood. (If the hood is itself pyramidal internally, some leeway is possible on entrance velocity because farther up where the effluent is potentially subject to reflection, the sectional area is less.) (Ignoring edge effects, the area of the duct times the air velocity in the duct equals the area of the hood times the air velocity of the air at the hood entrance.) Inside the duct, the velocity of the air should be in the range of 1000 ft/min to 2000 ft/min to keep the grease particles not captured by the baffles or screens from precipitating onto the duct walls. Going up in blower flow rate capacity (cfm) usually requires larger ducting, but this should be checked by calculation. For duct sizing, assume that an external blower with ducting, transitions, baffles, etc. will, at full power, pull about 2/3 its rated (zero static pressure) flow rate. For a more accurate value, one needs the blower curve (flow vs pressure), calculation of all the losses at every point in the path, and some knowledge of how make-up air and/or house leakage is establishing the negative pressure that the house will get to or that you will let it get to. If Viking makes a similar hood with an external blower, you should consider downloading the installation manual and noting how Viking directs the connection between power source, fan, and fan control. In general, the power connection to the hood control should have some spacing from the duct to keep the wire temperature within specification. kas

Thanks everyone. It's an open-air patio, not enclosed. The overhang does have vented soffit :( Also, I've never had a vent hood before. The smells of my cooking are not bad but they do linger, thus the hood. Boxer, what do you mean snake the tube? With a chimney style hood there can be no snaking until you get past the drywall. My problem is that there is a joist in the way. Actually, a ceiling joist, not a roof joist. We are trying to avoid cutting joists. Is there some way of snaking that I'm not aware of that will make a chimney style hood work? I know I can get an undercab hood or an insert and angle it within the cabinetry to avoid the hood. The chimney style would be better for the aesthetics of the plan and let more light into the adjoining room. I'd like to ask him to move the troublesome ceiling joist and reinforce to in order to fit a roof-vent chimney mount style, but that's where the roof slopes to its cramped little end and will be a &(&*^% to do.. I guess we'll see. He does like the chimney style as well, so I'll let him decide if he wants to deal with the joist or not.

Big caveat that we are new to this too so take anything I say with a grain of salt, but--- We had the same dilemma; our range is an old vintage stove and is actually quite deep (nearly 30" from wall). So I had been looking for a deep hood (27" in our case since we're doing a wall mount chimney hood), and found they were hard-to-find and pricey. The more we talked with various professionals, though, the more I heard that the depth is not such a critical issue. They recommended expanding the capture width if we were concerned about it, and most importantly making sure the CFMs and venting were adequate. Another recommendation was to try to cook really intensive things on the back burners. We ended up deciding to stick with standard depth hoods to cut costs...but since the kitchen isn't in yet, I can't say whether that was a good decision or not. We haven't settled 100% on our hood yet, but have been narrowing the field by going to local independent appliance shops, where they tend to have the hoods hooked up so that you can hear roughly how loud they are and how the lights/filters/etc. work. We also have a local appliance outlet that tends to have the best prices---worth looking to see if you have a similar shop nearby.

The cabinets over a range hood don't generally have enough room for "a bunch of stuff" by the time the ductwork is accounted for in them. If you assemble the cabinet correctly, and use glue, then it will be plenty strong enough to carry the weight of a vent hood.