I got Zephyr Hurricane (695cfm, 6" vent, though I have 3-1/4x10"). It's pretty cheap (under $500), has a good capacity as far as cfm, and I won't be doing any stove top grilling, using gridle etc., so capacity is more than enough.

I guess the rule of the thumb is to have 1cfm for each 100BTU of your cooktop. My Bosch gas range has total of 35000 BTU, so theoretically 350cfm should be enough.

I'm not sure how bends effect the needed capacity, my vent goes straight up so never was an issue for me but if you do search this forum, something should come up.

Yes, capture area is extremely important - with too small a capture area you will get grease escaping from the hood - in my experience the deeper the hood from front to back, the better.

If you have the room for the 8" duct, then that's what you should install - it will give you more flexibility with hood choice. Some people like VAH, some don't - but there are plenty of other 600 CFM hoods with large capture areas that will work with an 8" duct.

You should check out the appliance forum. There's a faq on ventilation and lots of threads about different hoods.

Like weissman said -- get as deep a hood as you can. We went with a FururoFuturo that has 940 cfm so I thought it would be great for my 36" DCS Range. Well it stinks because it's only 17" deep and if I cook on the front burners the smoke, and of cause the grease, goes straight up. Arghhh. We're thinking of eBaying it and getting something else even though we've only had it since Thanksgiving.

So -- go deep :-)

Liz

I agree- get the largest diameter outflow that you can. As stated above, you get more flexibility in choosing a hood, but it will also likely reduce noise. For any given CFM you can increase the amount of air pulled by increasing diameter of the pipe or the velocity, and a larger volume of air moving at a slower speed will be quieter than a smaller volume pulled faster.

Remember to consider how tall you are and whether or not you'll be banging your head into the vent hood.

The Zephyr Typhoon is 21" deep & max. height above stove top level is 30".

I just bought a Typhoon undercabinet hood and plan to mount it to the bottom of a shelf/ mantel affair. In my situ, the cooktop will be on an exterior wall with an easy out the back of the hood/ thru the wall exhaust. I'm thinking that I can add 2-3" of spacers behind the hood to increase capture area on the front side and not significantly compromise overall performance.

Capture area may be more important than CFM. Most likely you'll rarely have the oven and all burners going full blast at any one time. With a larger capture area you'll be able to operate the range hood at a slower speed for most everyday cooking, which will be quieter.

Two 90 degree turns and 12 feet of ducting is not an ideal situation, so to minimize noise and air resistance, you should try to oversize the ducting and make your 90 degree turns on as large a radius as possible.

I recommend Vent-A-Hood. I've been entirely pleased with the ones I have in two homes. If you go with a 600cfm model, you'll need a minimum of 8" ducting. I think you'd find that you operate it on the very lowest SensaSource setting most of the time, which is the quietest. Personally, given your venting length and turns, I might even consider a 300cfm unit but still install 8" ducting. Most likely, you'll seldom use a 600cfm hood at the highest setting, anyway.

I have a 48" undercabinet 600cfm model in my kitchen over a 36" cooktop. The large capture area is very effective with little or no smoke or fumes escaping. Because I have 15" deep upper cabinets, the hood mounted out from the wall by 3" with a stainless steel filler between the wall and the back of the hood, further expanding the capture area of the range hood. I'd estimate that about 99.5% of the time, I run it at the slowest and quietest SensaSource speed.

I have no particular expertise in HVAC, but I do know basic physics. For a given CFM there is an inverse relationship between cross sectional area of the duct and the velocity of the air. So for a 6" duct you need a higher air speed to achieve the same CFM as you do with an 8" duct. In fact, a 6" duct should require 77% more velocity than an 8" duct.

But the hoods are only designed to work at a certain range of velocities. I presume the hoods that quote 600 CFM with a 6" duct are designed to run at a higher air velocity than those (like VAH) that require an 8" duct.

In general I think one probably ought to follow the recommendations of the hood manufacturer. If the hood you choose calls for a 6" duct then its not at all clear to me what happens if you use a larger duct. It might be fine, or it might cause trouble.

Another factor is that there is a relationship between air velocity and noise. Its my impression that this is more important in the noise level of the hood than the noise generated by the blower itself. Other things being equal, a hood designed for a larger duct will probably be quieter than one of the same CFM designed for a smaller duct.

Nearly all hood filters only partially collect grease particles. The rest one hopes to eject from the house and not have collect on the ducting. The design range I've read for desired duct velocities is 1000 to 2000 ft/min.

Most ventilation fans are rated at zero static pressure, so the more resistance in the duct (smaller duct), the lower the actual air flow. However, it is easier to blow than suck, fan-wise, so VAH-like in-hood fan location schemes may be more tolerant of duct limitations. Fans closer to you (in the hood) may be noisier than fans on the roof or side wall, and I have read complaints about the VAH system having an awkward cleaning approach.

kas

w/smaller diameter ducts you'll have more noise for any venthood you select. Each manufacturer will have guidelines. We installed a Thermador quiet series 600 cfm(36" wide) over a 36" cooktop, it has 3 speeds. We were on the bubble at 6" diameter, but had the hole in the brick opened up to 8" diameter to be safely within specs.

I'll confess we didn't consider capture area, but when we have all 5 burners going on our cooktop (55,000 btu/hr total), this hood removes excess heat w/ no problem & everyone can have a normal conversation w/ out yelling. We haven't baked the finish off our cabinets in 5 years of heavy use. It looks nice & if memory serves it's in your price range - maybe a little over.