Hi Oaklandzev:

I am not a ventilation expert; but we did just go through a similar set of decisions when we recently bought and installed a new vent hood over a 30" pro style range (we ended up with a 36" wide, 24" deep, 1000 cfm hood, with a 90 degree bend, mounted 33" above the rangetop). I was so eager for excellent ventilation that I think I went a little overboard. But I'll never have to worry about grease and steam in my kitchen again!

Just a few thoughts:

-- unless you are getting a grill or griddle, 24" deep is truly fine. 27" deep seems like overkill.

-- with a vent that powerful I'd urge you to mount it at the height that feels most comfortable to you (and not to your contractor) since you are the one who is going to have to live with it; I'm about your height and think that our 33" high mount is just fine. No head bumping and still extremely effective. You could even consider bumping it up to 34" or 35" and my guess is that you will still have plenty of capture ability and suction.

-- my HVAC installer scoffed at the idea of insulation around the duct so we didn't do it. I guess I'll never know if it would have made a difference; but it couldn't hurt.

-- although going 6" wider than your rangetop for your hood will maximize efficiency, putting a 36" hood over a 36" range is also going to be just fine with the kind of cfm and 24" or 27" depth you are thinking about. Balance the extra width against other needs in your kitchen, including cabinet placement, storage, and aesthetics.

My experience:

36" Bluestar, lots of wok cooking, high-heat searing, etc.

Kobe CH-191 42" hood, 280-800 CFM.

When you produce smoke/steam, it billows up and _out_ quickly, so the larger and closer your "capture area" is, the more will get vented. Think of the smoke/steam as creating a "V" from the source, with a 45deg angle on the sides.

The CFMs necessary to suck out what's in the capture area are much less than what you need to get what's billowing out to the sides - to get what's billowing out you need a stiff breeze-like suction. Serious air movement. And the higher your hood is, the further out to the sides the stuff will billow before it gets to the level of the hood.

So having a deeper hood helps, as does having a wider hood. If your hood is only as wide as your cooktop, there will be more stuff that ends up underneath the upper cabinets to the sides that can deposit grease there.

The more you're depending on CFMs to get the stuff out, the more time before you start cooking you need to have the fan on to set up a good draft - especially with an external fan which is pulling, not pushing. If you forget, and your hood is smaller or higher, what gets captured until the fan can take it out is much less and that ends up in the room. Even though I know, I often forget to turn up the fan until it's too late.

Our hood is 42 wide by 22 deep, and we ended up lowering it to 27.5in above the cooktop after we used it a few times further away. For searing or wok cooking, the steam/smoke is short-duration and intense, so having the venting closer to the source made more sense for us. It also means that noise isn't really an issue, since it's only on high for a short period of time.

So you're losing some capture area by going 36" wide, and adding some by going 27" deep. 27" deep is definitely going be something you are banging into, and going with 42" in order to raise it makes sense to me.

The larger the capture area of your exhaust hood, the fewer CFM's required to effectively remove cooking fumes.

I have a 36" gas cooktop in my kitchen with a 48" range hood that extends out 27" from the back wall. It's mounted 27" above the cooktop. I'm 6'1" tall, and I have never bumped my head on the range hood. As long as the hood doesn't extend out from the back wall further than the front of the countertop or range, it seems unlikely to me that anyone would bump into it, regardless of its height.

I operate my range hood at it's lowest speed setting (approx. 200 CFM) about 99% of the time. Virtually no fumes or smoke escapes it. I know this because I have recessed can lights throughout the kitchen and I'd be able to see errant smoke or airborne grease in the beams of light - if it was floating around. I also don't have even a trace of oily film on anything in the kitchen after nearly 5 years with this setup. I'd urge you to go with the largest capture area you have room to install.

Just because you have a blower capable of moving 1200 CFM, doesn't mean you're going to be moving that volume of air. Have you thought about make-up air? Whatever volume of air you're exhausting has to come from somewhere. If your house isn't leaky enough to allow 1200 CFM to be pulled in around windows, doors, ceiling light fixtures or other potential sources of outside air, you're not going to be exhausting 1200 CFM, no matter how strong the blower. You could end up back-drafting fireplace chimneys, or worse, the flues on other gas appliances in your home.

That being said, I really like the Fantech exhaust systems. They are very effective and quiet. I think you'll be very pleased with that choice, but I would definitely urge you to rely more on a larger range hood, than massive numbers of CFM.

We have only put ours up for height checks.

It really makes it much easier to use if you have no problems reaching and seeing the respective buttons when you are in front of it cooking up a storm with one hand! Please take a look at where the buttons are in relation to your height. If you can't do that easily, you will be less likely to turn it when you need to and will be distracted from what is happening on your cooking surface.

I also would reconsider the 27" deep because that will stick out past your countertops. If you really are wanting that much depth, then you might look at Thermador among others who make a slide out vent that goes to that distance and retracts when you don't need it. Also, most cooking surfaces are set back a bit, so I wonder if you really need to go that far forward.

IMO, I think you might be over doing it and unable to use the maximum power of such a unit because of the make up air issue. YCMV!

I don't think you stated whether there would be cabinets on either side that would be lower than the hood bottom. If so, then you could go with a narrower hood (36") since those cabinets act as 'capture'.

IMHO, the main noise comes from the blower. Since yours is remote, you should have less noise issues.

Do your cabinets go to the ceiling? if not, will the vent go across the top of the cabinets? If so, could you enclose it with a soffet? That would take care of any noise issues.

Also, be mindful of the distances. The 90* elbow adds an effective distance of something like 15'.

Thanks for all the input, everyone.

So far it sounds like having the capture area larger is warranted so I won't bump my head and will still get the stuff that is billowing out.

We will have cabinets on both sides, and they will go to the ceiling.

I will have to talk to our contractor (when we pick him or her) about the makeup air. Our house was built in '39, and we are in northern CA so we have "California coolers", which are vents in our cabinets along the wall with shady exposure, where the kitchen is. I want to keep the ones under the counters to reduce odors from the trash bin and figure that could also be a source of make up air. But we replaced our windows and doors years ago with double-paned so the house is otherwise pretty air tight, and I am concerned about sucking in from the chimney, or just not getting CFM. I'll keep that in mind for sure. And currently the cabinet doors under the sink are not solid (not quite a mesh, it's a pattern of holes) to let the air circulate. Our new cabinets would probably have to be customized with soffit vents to enable that airflow.

Update: my KD was here today, and she thinks that with 1200 CFM and 42" wide we can go with 24" deep and 33" high with no problem. She says we can get the make up air as I suggested. We'll wait for the contractor for final say. Thanks everyone!!

I don't have time today for a long discourse on effluent expansion, but mondragon is exactly correct; the effluent expands as it rises, and no exhaust fan likely to be used in a residential setting will pull it into the hood as it passes by it. You then have to wait until enough household air changes have occurred to get rid of it (or the part that didn't precipitate on some surface). Has your KD bothered to read the literature on the subject that is scattered about the WWW?

kas

1200 CFM sure is a lot of air to be moving. That's as much as a standard 3 Ton central AC unit! 1200 CFM would completely exchange the air in a 15x10 room in just one minute. Why do homeowners go after these extreme hoods? I think it is excessive, but maybe it's fun to have I guess, and hey, you don't have to always operate it on high right? But it is certainly not worth worrying about.

Also, as someone else pointed out, there is the need for make-up air. If 1200 CFM is going out the exhaust, then 1200 CFM has to be coming into your house from the outside somewhere. If you don't like the climate outside, you're about completely replace the air in your house with it in about 15 minutes ;-) Food for thought.

We have a 42" wide & 24" deep hood mounted 30" above our 36" cooktop. We're a tall family...ranging from 5'10" to 6'6" and still growing! So far, the only time anyone has hit their head on the hood is when my DH stuck his head up into it to check out the "squirrel cages" and didn't bring his head down far enough when he exited...he hit the back of his head!

I don't think you should worry too much about head-hitting. Mounting your hood within the manufacturer's recommendations is more important.

As to 1200 cfm, it sounds like you won't need makeup air b/c of the "California Cooler" ventilation.

What some people don't understand is that the higher cfms have many pros: If needed, you have the higher power to remove anything from the kitchen (including smoke from burnt toast! yes, experience!)
You will be able to run your hood on a lower speed more often and the fan will be quieter running on low or medium than on high. With lower cfm hoods, you may have to run your hood on high a lot more and the fan is generally louder than a higher-cfm's fan running at a lower speed w/equivalent cfms.
The more bends you have to make in a duct run, the lower the actual cfms are. So, if you start out w/low cfms, any bends (or excessive distance) will lower the actual cfms even more.

You won't be moving 1200 cfm with a 1200 cfm blower. That is the rated output at zero static pressure. However, the actual static pressure is some negative value when ducting, duct transitions, and hood baffles/filters are in the air path. Maybe 800 cfm would actually flow at full power with a wide open house, depending on many factors.

One factor is house leakage versus make up air availability. The less make up air or the tighter the house, the more negative the pressure gets in the house and the cfm drop even further. But the flow pulled by a 1200 cfm-rated blower will still be higher than a 300-cfm rated blower could manage under the same circumstances.

kas

OK, sounds like the consensus is that with the make up issue addressed, I'll be able to do what I want, which is use it on low most of the time so that it's super quiet and with 1200 CFM in the motor I'll get an effective CFM that will work.

buehl - thanks' for the reality check. If 30" works for you, it will work for us.

My contractor said he needs to mount motor on exterior wall because
attic too much going on, rafters, etc. I am concerned that the exterior
wall still will bring noise unless maybe 15 feet away, etc.