FWIW the induction site says you need 100cfm for every foot of induction cooktop. IMO this is too low, at least for my kind of cooking which involves stir-frying and searing. We installed a 720cfm Kobe which handles smells and aerosols from cooking on the 36" cooktop very well. Our wall oven is on an opposite wall, and we get more food smells from that than from the cooktop.

Our kitchen is part of a large open area which includes a living room, family room and dining room, so kitchen smells spread through the entire space.

Cheryl

>>>"All the calculations to figure out the required CFM take into consideration the size of the kitchen . . ."I'm interested to hear how was this done. Or, maybe, you are are talking about make-up air (MUA)?

>>>"My cooktop is a Bosch 36" 800 series induction, so how do I take that into account when it asks for BTUs in the calculations?"

Lots of prior posts here on this subject, by the way.

Two rules of thumb for determining CFMs.

(a) 100 CFM per linear foot of cooktop (often used for electric cooktops and stoves including induction.)

or

(b) 100 CFM per max BTU-hr rating of all of the burners. To convert electrical/induction burner kW ratings to btu-hrs, multiply by 3412.142. Divide that number by 100 and you have a CFM rating.

Both rules of thumb will say you can get by with a hood that only pulls about 350 CFM for your Bosch. For quiet operation, however, you want something with at least double that. A more powerful hood, run on low to pull 350 CFM generally is going be quieter than a less-powerful hood run on its max setting. Also, as Jadeite says, you sometimes want the greater power, as well.

If you were not finding the prior discussions here with the GW search engine, try doing the search using Google. I would suggest starting with these terms: "rangehood + CFM + island." Doubtless, you can think of more terms to use.

This post was edited by JWVideo on Sat, Jan 12, 13 at 12:59

Jadeite - thanks for the direct experience, that helps. Did you need makeup air for your hood?

JWVideo, Thanks for the explanation! I looked through previous threads, and I still end up confused...

One thing I read that you are supposed to take the size of the kitchen into consideration. The calculation was to find the volume of air in your kitchen (length x width x height) and then divide by 4 to get the CFM rating. For my kitchen that works out to 665, but since it is open on one side I was wondering if that implied I needed more?

However another calculation said to find the volume of air in your kitchen times it by 15 (to change the air 15 times in an hour) and then divide by 60 as CFM is measured in minutes, then add 100CFM for every 10,000BTU and then another calculation to add for the length of duct which I found to confusing to even repeat. I did read this on an old thread that I now can't find, but here is a link to another site explaining it.

Here is a link that might be useful: Calculate CFM for range hood

Okay, I see the confusion.

The linked site seems to be mixing up concepts of (a) makeup air (MUA) and room venting into (b) trying to figure baseline minimum hood capacity while (c) throwing in a formula for off-setting efficiency losses caused by bends in the vent ducting (bends reduce the effectiveness of airflow.)

To me, the air-change calcuation seems a bit misleading. It looks more like the kind of calculation used for a room venting fan --- say, a ceiling vent fan as for a steamy bathrooms --- rather than a range hood over a cooking appliance which is intended to vent a specific small part of the room. The HVI standard is a suggestion for very tight homes and is not code. FWIW, the 15 air-changes per hour is for the specific kitchen area itself and even the large room you willhave is not going to require very much venting power. So, for practical purposes, you do not need to mess with this for the range hood because any hood with at least 600 CFM will be more than sufficient.

But there are a several other things for you to consider and those may lead you to a larger hood and a better appreciation of MUA.

Number 1, a newly constructed house is likely to be pretty tight. Are there local code requirements for a MUA system? Even without code requirements, MUA for a new house is important to prevent backdrafting of appliances like gas water heaters, HVAC furnaces, and fireplaces/woodstoves. Adding extra-power to your rangehood can complicate that. Bathroom fans are typically below the threshold for needing MUA and risking backdrafting, but rangehoods can be a lot more powerful. Fortunately, adding MUA for the other appliances can be pretty simple and inexpensive. There are also relatively inexpensive simple MUA vents for kitchen rangehoods in warm climates. This is something to discuss with your builder.

Number 2, home air exchanges are a whole house issue that require addressing when you have a new and very tight/very energy efficient house. It is a separate consideration and, if you live in a cold climate, you may need an air exchanger regardless of how much of a range hood you have. As far as having air-changes in the kitchen, you've got the adjoining great-room space. The linked site's kitchen air change calculation assumes a confined kitchen which has to draw air from other spaces, but the large space you have avoids that as a problem, I think.

Number 3, look at range hood design. It sounds like your stove/cooktop will be going against a wall where you can have a canopy-type range hood. These are more efficient at gathering vapors, excess heat. etc. than the stylish but less efficient flat-bottomed and curved-glass designer vents. If you will be using one of the latter, you probably want more CFMs because they are less efficient than the canopy units at collecting the stuff that needs venting.

Finally, the general rule of thumb, as you probably know, is that you want a hood that extends three inches to either side of your cooktop in order to better capture the steam/smoke that may be spreading horizontally. This is a suggestion and a rule of thumb, not a requirement, of course. A 36-inch 600-700 cfm canopy hood (such as a Kobe) would probably work very well. You might want a bit more CFM for a flat-bottomed style of venting hood.

So, talk to the builder about MUA and code requirements, and think about the design of the hood you are getting.

Thanks, that does clear it up somewhat!

I emailed my builder and he says I don't have any local requirements for MUA (I'm in California), though if it is needed I'll put it in, though given our climate we don't need a home air exchange.

I'm not sure we can git a 42" hood in the space we have from a functional point of view, but I will get one that is chimney shaped, and not a flat glass flat bottom type.

I was planning on going a little over what I need in CFM so I can run it on low most of the time, and because I'll be getting a 36' instead of a 42'. I was looking at the 1000 CFM models, but after this conversation I'm realizing that isn't needed at all!

Thanks!

hmm... so I'll have to talk to my contractor about MUA options, though I do live in a very mild climate (California) and opening a window would seldom be a problem.

This is seriously the single most confusing item in kitchen planning!

While you mentioned ducting, there was no mention about the diameter of the ducting. Make sure it is 8" or 10" diameter. If your duct is smaller, you will be pulling air through too small of an opening, and you will get increased noise and decreased efficiency, regardless of the hood you install. Don't let your builder tell you it's not a big deal, or scoff at 8" ducting. They often do that cause it's a bit more difficult for them to install something bigger than a 6" diameter duct, so they'll try to get out of it. Meanwhile, you will have to live with it. So make sure your scope of work with the builder stipulates an 8" or 10" duct. The specs of the hood you choose will tell which size duct it needs.

This post was edited by akchicago on Sun, Jan 13, 13 at 13:45

Laura12:

Do go look Kas's clippings. Lots of very useful info if you want to delve into understanding the art and science of range hoods and kitchen venting.

Also, use his screen name as part of a search-term string and you'll find the most pertinent prior discussions here, and those may help dispell some of your confusion about how things work.

Lots of prior discussions here about sizing ducting, as well.