I have so often wondered about this! I live in the north, where much of the year you can't just open a window to let more air in. Our code says that you need make-up-air at anything over 300cfm, and this limitation (mentioned by my kitchen designer, contractor, and appliance salesperson, to their credit) drove my choice of a range away from anything too high-powered. But what about places in the south where for much of the year it's too HOT to open the windows?

Gee mnerg, at least your appliance guys were honest enough to acknowlege the MUA issue. Mine were too afraid of losing sales on high powered ranges and fans.

FWIW, I had to install heated MUA to go along with a 600cfm hood. This system cost more than the range and hood combined (add in a TOL Miele DW and it almost balances...). I'm always boggled by the contributors on the appliance forum who recommend installing higher cfms so you can run the fan on quieter low settings. But I guess some remodellers have unlimited supplies of $$$, while most of us do not.

The author said, "...most residential kitchens are adequately served by a 150-cfm or 250-cfm range hood..." What he didn't say, I don't think, is what size home, family, and what size/power/type of stove fit this generalization. I had a 525 CFM hood in our last house over an electric ceramic top stove. I did much less cooking in volume and the amount of heat then, and although it did an OK job, it wasn't perfect at removing all odors, etc. Is the author trying to accomplish the same thing with the 150-250 CFM hoods that I hope to? What is 'adequate' to him?

I'd like to know the truth about the number of CFMs I need for the stove I have to cook for my large family, and how to make up for that, if needed. I was just left a little lost and confused after reading this, but almost certain I have too strong a hood for my house. I guess we should just be glad the pets and kids run in and out all day, so the air is exchanged some that way. And we don't have a woodstove or fireplace, but after last week's 4 day power outage, I've been seriously thinking about adding one, so this becomes more of an issue.

**The author said, "...most residential kitchens are adequately served by a 150-cfm or 250-cfm range hood..." What he didn't say, I don't think, is what size home, family, and what size/power/type of stove fit this generalization.**

I read "most" as meaning... I can't think of a synonym, so let's just say "most" again. As in, most people have 30" four-burner ranges (or smaller), rather than six or eight-burner high-end ranges; most households don't have more than 4-5 people in them; most homes are X square feet or less (I'm sure that number is easily available somewhere on the web). So, for most people, 150-200CFM is plenty.

And even for those who do need more CFMs, still, most people with high-powered ranges never use all six (or however many) burners at the same time, and even when they're using multiple burners it's usually not all for high grease/high smell foods; the highest-powered burner might be cooking the spaghetti, for example. So we need fewer CFMs than we think.

Here's some reading.

Note that all the links below are in the context of a commercial kitchen, where there is high-volume frying, searing, woking, broiling, etc going on almost all day. In our home kitchens, that sort of cooking is intermittent and usually pretty brief. So perhaps a few minutes of unconditioned MUA is tolerable or, at least, its not worth spending big $$$ to heat/cool the MUA (unless an inspector requires it).

Also, the big-CFM exhausts are really pretty darn big relative to the size of a typical home kitchen. My kitchen (admittedly a little one) is 1800 cubic feet. A 900 CFM exhaust hood would theoretically empty that entire volume in 2 minutes! This makes me think that, if the hood is well designed (efficiently capturing the rising effluent from the rangetop), there really shouldn't be an frequent need to run the exhaust on "max" for very long at all.

So, why do we read on the appliance forum and elsewhere that a 48'' pro-style range needs seventeen billion CFM of exhaust plus a screen to catch all the toupees that will be sucked off your guests' heads? Those formulas usually count the total BTU/hr that all the burners and all the ovens of the range can produce if they are all on ''high'' together. How realistic that is, is a worthy topic.

http://www.energy.ca.gov/reports/2003-06-13_500-03-034F.PDF

Most informative guide to MUA systems that I have found so far

The solution is to specify an independent makeup air supply. But, once dedicated MUA has been added to the system, the challenge becomes introducing this air into the kitchen without disrupting the ability of the hood to capture and/or without causing discomfort for the kitchen staff. Kitchens are not large and dumping 7000 cfm of MUA, for example, in front of a cook line does not go as smoothly in practice as it does on the air balance schedule! Not only can makeup air velocities impact the ability of the hood to capture and contain cooking effluent, locally supplied makeup air that is too cold or too hot can create an uncomfortable working environment. This design guide presents strategies that can minimize the impact that the makeup air introduction will have on hood performance and energy consumption.

http://www.energy.ca.gov/2005publications/CEC-500-2005-138/CEC-500-2005-138-FS.PDF

Seems to just reference the above report

http://www.energy.ca.gov/title24/2008standards/prerulemaking/documents/2005-10-24+25_workshop/2005-10-24+25_LBNL_RES_VENTILATION.PDF

Mentions MUA in a survey of relevant codes:

Many codes require makeup air to reduce negative pressure caused by exhaust fans or testing for spillage. The ALA Health house simply states to “reduce the depressurization of the building.” ASHRAE Standard 62.2 states that the net flow, at full capacity, of the largest two exhaust fans cannot be greater than 15 cfm/100 ft2 of occupiable space. If this condition is not met, compensating airflow must be provided, maximum flow reduced, or atmospherically vented combustion appliances inside the conditioned space must be tested for spillage. CA title 24 requires that glass doors and an air inlet be provide for each fireplace and each wood, pellet or gas stove. The Minnesota code limits "excessive depressurization" except when all appliances are sealed combustion and 3 biggest exhaust fans have a combined flow less than 425 cfm. There are 4 prescriptive paths or a performance test to show compliance. The Canadian code requires make up air for exhaust flow greater than 150 cfm if there is a chimney vented oil or gas appliance in the house. The EPA IAQ standard goes even farther to require a direct vent or power vented water heater. It also requires a direct vent furnace if the house is located in a climate with more than 4000 Heating Degree Days (HDD).

Even 4-burner ranges can range in power quite a bit...and it was mentioned that house size plays a role, and, I believe, standard home size can vary a lot from region to region. Just saying, I wish there'd been a little more info as to specifics. And I still wonder what 'adequate' accomplishes...Complete capture of grease,fumes, odors, and steam/smoke? Or just enough for some sort of safety or clean air margin?

But anyway....Now I'm with JohnLiu and am wondering, big time, about the 'formulas' given on the appliance forum about figuring 'needed' CFMs.

My brain has turned to much on this subject now, so this may be a completely idiotic question: I know there could be air quality issues with smoke from burning food oils, and eventually possible mold issues if moisture isn't vented, but is there a safety reason for running the fan when using a gas (LP) stove in terms of getting rid of the combustion biproducts/exhaust?

This is all making my head hurt. Ack!

So I just ordered a 760 CFM hood (that can also be run at 240, 420, and 580 CFM) to vent a 62,000 BTU all-gas range in a kitchen with a 1400 cubic feet cooking area (though the actual room is 2100 cubic feet if you count the non-cooking end....) I went by the little guides that say 100 CFM for every 10K BTU...but maybe that was wrong?!? (The house is tiny, but I can't see how that relates to the draw of the hood, as the article seems to suggest---isn't it more about cubic feet of air movement?)

From all of this, it sounds like maybe that was overkill?? I just ordered it Sunday, so I can probably still change it, I suspect....are we paying for CFMs we don't need at all? Augh.

here is what I wrote on the other thread about range hood size-

I can answer the question as to why you need to get MORE cfm's than you think you need. The reason you run the unit lower than the high has to do with the physics of the air in the duct. When you run the unit on the highest setting you create turbulence inside of the duct. This in turn leads to the HOGS returning to your kitchen " back at ya" as it were , rather than venting them out. This is a fact. This is why I have 1400 cfm in a 54" hood liner. I use it on medium and low. Mine is quiet even on high but I don't need the high setting. I have NEVER had to clean one spot of grease off of my wooden hood or the surrounding cabs/shelves. There is never any residue ever at all. I do have direct splatters on the wall behind the cooking pot at times when DH woks but other than that the HOGS are all removed completely.
I read the article sited and had a problem with a lot of what he said. I don't have any unsealed fireplaces in my home and my waterheater is outside on the wall of the house . Since our house is an 1890 the problem of drafts and makeup air doesn't exist for me , which I am glad of . As far as I know there is no code here, or wasn't at the time of my install. I am glad of that too. We need the cfm's that I have since I see the result on the baffles every single Sunday when I clean them...they are all gresy greasy gressy. I also can't tell what is cooking in the kitchen at all even sometimes when I am standing in the kitchen. It sucks LOL.

The other thing that bothered me about the article and the backdrafting etc was that the implication is that you run the hood for great lengths of time. In all of my experience that is not the case. Wok cooking requires minutes as does deep frying. I never have it on for more than possibly 30 min. and usually way less than that.

So that is of course my experience of 4 1/2 yrs with this set up of a Tradewind LIner. I would like to hear from others. c

rhome410! I just posted this same article before I started reading tonight! So I guess we got it covered.

Hi, C!!!
I have been making stock lately, which means a pot on the stove all day, and when I make stews, soups, and some sauces, they cook for hours, so the hood stays on for as long as the rangetop is on. I've always understood it wasn't safe to run the stove without the vent on, even though we had to during the power outage...which made me uncomfortable, especially with most of the deaths during power outages around her are due to CO2. (Of course some of those are people who will actually burn a charcoal bbq INSIDE their home and other such practices.)

So are you saying that you get a more powerful hood just to get a larger duct, which has the benefit of better efficiency and less noise? Is it necessary to pay for more power just to get the larger duct?

I can see that if you don't have to worry about the make-up air issue, that makes the point of this article moot. But I was already concerned about not having make-up air for our HVAC system and now this could exacerbate the potential problem...?

First it is CO, carbon monoxide, that you would worry about. CO2 is what you exhale :)

I don't know where they got the info for telling you that you must run your hood all the time that your gas range is in use , I also don't know where the idea came that there is ANY danger at all from CO and your gas HVAC. This is a case of too little knowledge on the part of the people selling a product and misplaced fear on the part of the homeowner.

The only way you get a buildup of CO is incomplete combustion, or an actual gas leak.Your gas range is designed to have essentially almost complete combustion . So is your HVAC. Your gas range is NOT depending on your hood to be removing toxic waste to prevent you from dying from CO poisoning. A gas water heater is vented as are all of my gas fireplaces. You should have a CO detector in the rooms with gas fireplaces and then one in your kitchen. There is enough ambient air in your kitchen that it is impossible for there to be any CO buildup...impossible. If you had a tightly sealed tiny room with the gas turned on and NO flame and you stood there and breathed it,,,yes you would get sick and die. But that can't happen as long as your appliance is working correctly...the flames are adjusted correctly and the there is no gas leak.

The reason people die in cold weather from CO poisoning is that they build large fires in very small rooms and the O2 is overwhelmed and can't convert the CO to CO2. You do have to have venting but in a normal home with functioning appliances there is " 0" risk from from CO.

The only reason I have my larger duct/hood is so I can remove HOGS....heat, odors,grease,steam...you note that there is nothing in this about toxins....it won't happen and isn't necessary. Hope this wasn't so long a diatribe that I was out of line. I just get crazed when I see this stuff and DH said to write this and maybe set it straight. c

Not even taking the hood into account, anyone with a newer home should really have some form of makeup air in place. Especially for those in very cold climates where they house stays sealed up for months.

Back on the hood issue, I have a 600cfm ventahood and when I stir-fry or sautee it's not really adequate to the task. It's mounted higher than recommended which doesn't help, but I wish I'd gotten 900 or 1200 cfm. And that's just using a single burner.

In reality though, ask yourself how many times you would be using more than one burner on high for more than a few minutes? This is why I'm not really on-board with the idea of calculating cfms based on the total range capacity. Anyway, gardenwebber's suffering through a chilly winter have my sympathies :-)

In response to trailrunner - I have used a prestige 1600 cfm 54" vent hood over my 48" bluestar range since March. It's vented thru the side of the house across the pantry and is quieter than my stupid old recirculating fan. (I wonder if it's working to it's full potential since it's so quiet) There are no doors thru the kitchen, dining room, sitting room, 3 hallways, and up 2 sets of stairs to the 3rd floor (maybe 1200 sq ft with 9' to 10' ceilings?) plus 20 windows, 5 exterior doors and 2 fireplaces in this path in an 1825 house (100 - 150 year old windows). Not all interior doors will be closed off to other rooms so more rooms, windows and fireplaces (11 total) will be added to this space.

I don't think I need make up air. Both new furnaces and the water heater are directly vented outside and right now we use the fireplaces for candles only. We will line one for a gas insert but no plans for wood. My vent does it job. The bigger one was only a few hundred dollars more so I thought to get it and not work it so hard. Yes, I have had 6 burners going and broiling in one oven plus the other oven at once (but not the grill). During these times I'm usually burning the garlic bread in the broiler and that fan does a wonderful job wisking the smoke out (in my old set up 3 smoke alarms would be going off). We have 5 hardwired CO detectors too.

Oh, duh, sorry on the CO vs CO2 thing. Of course I know that! I said my brain was mush. ;-)

I'm sorry I wasn't clear. We don't have gas HVAC...But it has no fresh air inlet, which some say it needs and some say it doesn't. (issue Chiefneil is mentioning) So like all of this, I am clueless and the differing opinions and technical information has my head spinning.

You did NOT go on too much, Trailrunner. Thanks for taking the time to clarify things...and thank dear R for his encouragement in setting things straight!

I'm going to go back and put my head in the sand and decide what I have is just fine...a 48" wall hood over a 48" rangetop, but with graduated cabinets that probably help capture some of the HOGS. ;-) But if I want to capture those when I do slow cooking, it is on for hours. I run the hood mostly on low, unless stir frying or searing, but I can't find out what CFMs that is. I've forgotten what I knew when we chose it and all I can find now is that the max is 1200...and that the blower has 'infinite controls.' It doesn't go down to 0, I know, but I can't find where it stops.

I thought maybe part of the answer was opening a window just a crack, but the manual, which doesn't tell much, says to close the windows to have it work the most efficiently. I thought maybe it would help it draw.

Artemis78, did you check your local laws and rules? Where I live, any hood more than 400CFM *REQUIRES* a make-up air system. I can't pass the inspection without it. Therefore I am getting a hood that maxes out at 400CFM. You can check this with the planning department or whoever it is that you get your permits from. It would be wise to check it ASAP, while there's still time to cancel your order.

Personally I think there's a lot of overkill, and a lot of over-eager salespeople, in the vent hood... uh... industry, if that's the word. When are you ever going to have all of your burners going full blast? Probably never, and even if it does come to that, a less powerful vent hood plus opening a window or two would probably be all you need. I can't tell you what CFM is right for you, because I don't know how you cook or what your local rules are, not to mention how else your kitchen could be vented (how many windows...) or where your stove is. That latter point matters because a hood that has, say, 300CFM but vents striaght outside with only three feet of tubing will probably give you the same result as a hood twice as powerful that snakes its way up to the attic before going outside.

Thanks---yes, as far as I know we have no makeup air requirements in California. (Go figure, since we have every other requirement!) I can call and triple check tomorrow, but definitely no mention of it by appliance shop or inspector at the rough inspection, so hopefully we're clear there. We're in an old house that's very far from air-tight, though (just had the blower test done this fall!) so from that perspective I'm not too worried about it---also have two windows within reach of the stove, so that would be an easy fix if we did have air flow issues. Luckily the hood vent is straight up through the roof, so not too much impact on air flow there.

Mostly just wondering if our money would have been better spent getting fewer CFM but a wider hood....argh. Ah, well.

Interesting...here's what California code has to say on the matter.

Current code:
Makeup air requirements for the operation of exhaust fans, kitchen ventilation systems, clothes dryers, and fireplaces shall be considered in determining the adequacy of a space to provide combustion-air requirements.

New code as of January 1st (not applicable to our project):
Where exhaust fans, clothes dryers, and kitchen ventilation systems interfere with the operation of appliances, makeup air shall be provided.

Talk about vague! So I still have no clue if makeup air is needed, though I now know where to find the calculation for it, if it is...

No, trailrunner, your "diatribe" wasn't out of line. It's an important point (as I understand it) that a high-BTU range only "needs" a good vent fan because it's capable of the sort of cooking that throws off lots grease and steam that one would want to clear, not because it's dangerous without venting. The backdraft danger comes when low pressure in the house causes OTHER combustion sources (notably, fireplaces) not to exhaust properly.

I also saw the article and read all the comments after I received an email from Fine Homebuilding. Thanks for posting it Rhome.

The writer is an expert on building science but perhaps not on top of the kitchen issues (but I agree with most of what he writes in other articles and posts). No one runs their 1200 CFM fan continuously and it's usually not a big deal to open a window, especially at the price of a make-up air unit!

I researched this issue a while back and there weren't many solutions out there. Based on what I recall some of the best (and pricey) make-up air systems sucked the outside air into the return air duct of the HVAC system for the house and let the HVAC unit do the heating/cooling of air.

That being said he is correct that make-up air is being ignored as an issue and there aren't really any good off-the-shelf solutions for big hood fans in residences. This will be an area to watch.

Billy

Wow such helpful comments! We are installing a GE Cafe dual fuel range, 5 burners (which I doubt we will ever use all at once), and we have a 30 inch 600 cfm "Max" (with 3 speeds, so 600 cfm is highest speed) hood coming (Zephyr brand Tempest I). Any comments or suggestions? Our house is just shy of 3000 sq ft, plus attic and basement almost doubling the sq ft, so I think we'd be OK even though we do have a chimney vented gas hot water heater.
I prefer not to kill my family with this hood!

Finally! I tried several times earlier to read it but the site kept timing out! We must have all been trying to read it at the same time!

I admit I didn't get much past the first page of comments, but "interesting" is a good description. Personally, I don't agree about the supposedly max cfms and hood sizes the author thinks are adequate, but I do agree about the makeup air (MUA).

I will say, though, that we do not have MUA even though we have a 600 cfm hood and have not noticed any issues with air being drawn from the fireplace/chimney or even the oil furnace venting. It may be that we don't have a very well sealed house even though it was built in the mid-90s! (I suspect that is the case as I do feel a draft in our FR and DR near the windows on occasion. Replacing windows is on our "to do" list...but not for a few years yet!)

Well, all I know is that I have a Kenmore/Whirlpool gas range with a max 14K Power Burner that I use for 90% of my cooking. The Broan Allure III, which maxes out at 300 cfm, is a wimp and totally inadequate. Grease drifts across the counter to the left of the stove and there's always a disgusting film trying to build-up on the top side of the vent hood.

Part of the reason is no doubt due to the out-the-back vent pipe, which reduces efficiency. I have an open living plan, and if I cook burgers on my grill pan on the stove, afterwards I have to run that Broan at least 90 minutes on full blast to avoid smelling burgers in my living room when I sit down on the sofa after dinner. And high speed is way noisier than the 3 sones they like to publicize, which is only applicable to the 'low' setting.

What really astonishes me is that my original vent hood, a $79 cheapie Kenmore/who-knows-who-made-it, was sooooo much better than this Broan Allure III. Sheesh!

Thanks for this set of postings. I've been trying to connect this conversation to a pamphlet by Minn. Dept of Commerce about backdraft and sufficient air in a house entitled "Combustion & Makeup Air" but the URL for a pdf won't work in my post so I'll quote the pertinent lines:

"Combustion air problems can occur in any home, even older homes which were once assumed to have enough air leaks to provide outside air for combustion _any home improvement or weatherization measure (such as tightening up air leaks, adding insulation, or replacing windows) will increase the potential for backdrafting._ So, too, does the recent consumer tend of installing sophisticated cooking appliances with built-in high-volume exhaust fans. These fan/exhaust systems are powerful and remove more air from the home than what can be supplied through leaks or passive openings. Because dedicated makeup air is needed for these devices, kitchen exhaust fans with a design capacity greater than 300 cfm are strongly discouraged.

"Bigger is not necessarily better when it comes to kitchen range exhaust systems. In order to be effective 'downdraft" exhaust fans mounted near the cooking surface must have a large exhaust capacity (600 cfm). A modest sized (250-cfm) hood exhaust will be just as effective at removing odors and pollutants.....

"In homes with large exhaust appliances the Code may require a powered makeup air source."

There is reference to a software available from the State to help make calculations. The pamphlet says to discuss your situation with your local building inspector.

Somewhat off topic...
Tighter houses are becoming an issue for mold also, another reason for plenty of air passage so that moisture doesn't build up in winter homes in north. It is my hope that our pre-1980 wood stove and our crummy, leaky old fireplace are sufficient for makeup air, now that we've installed good windows. I just checked my Broan Allure III info...It's 300 cfm in regular modes, 430 cfm in boost mode (intended to be used for a short time--see below). We use it over an electric range. We have geothermal heat so no need to make up air there, but we do have gas water heating. I've heard that stronger fans might suck out the flame on the pilot light.

Off topic but might interest some...
Today I nearly set a paper-enclosed muffin afire in the microwave--must have punched the wrong buttons. The mic is across the room from range hood. Boost mode didn't work fast enough--given the distance, it might never have been helpful. I had to open window by mic and the outside door in adjacent room before I got enough air movement to remove much of the throat-scorching smoke. I had the boost mode on for an hour afterward with the window open but the door closed because it's cold outside and I wimped out. Despite the manic airing out, it will smell like burnt popcorn in here for weeks, I fear.

The next time we have a fire I'm going to check it with and without the vent hood on...I want to see if our vent hood is drawing MUA from the fireplace chimney or elsewhere. Hmmm...I need to check the oil furnace as well, I guess.

Thanks Rhome! Now I'm even more confused about CFMs, hoods, MUA! I already had a headache from studying this last week! But maybe I've just learned that I don't need as many CFMs as the equation often quoted here says.

I just open a window when I have the hood running at high speeds. Sometimes it makes the kitchen feel drafty, but I have a closed off kitchen and I would rather pull outside air in and up the hood than have it sucking heated air from the rest of the house out. Our boiler is two floors away and a sealed, direct vent gas unit, so I am not worried about backdrafting.

I live in New England, and while I love to save money and energy, I believe that it is very unhealthy to leave your house all sealed up tight all winter. I crack windows on warmer days throughout the winter, and I always crack the bathroom window when showering and leave it open for an hour or so afterwords. Maybe it's some kind of phobia. I compensate by setting my thermostat quite low, between 55 and 60. Our bills are reasonable, and the house feels fresh.

Nice if you can crack a window open to provide air to your fan. Mine have been frozen shut for two weeks now. Even if they thaw a bit, I don't want almost-freezing air blowing up my backside while I'm cooking....

It's important in cases like this to check the biases of the people doing the writing. These folks are approaching the problem from a stated priority of the primacy of energy efficiency, passive houses (i.e., ones that don't require energy for heating and cooling, but use mass, shade, sun, etc.), and other environmentalistic factors. The author said the following in response to someone who quoted the GW line on hoods. Note how it dismisses the need for getting rid of HOGS if it means using more energy.

You are correct that a 700-cfm fan will do a better job removing grease, smoke, and cooking odors from your home than a 200-cfm fan. But there is no getting around the energy penalty. If you operate a 700-cfm fan during the summer in Houston, you will inevitably be pulling in 700 cfm of hot, humid outdoor air into your house. One way or another, that outdoor air will need to be cooled by your air conditioner every time you operate your exhaust fan.

Most participants in the discussion admit that the simple solution if someone wants to adequately vent is to open a window (and let the HVAC do its thing on it eventually). Some, who work with high volume hoods, have good ways to provide adjusted make-up air without oddities or bother (but, of course, money to do it right).

Even though I am very interested in conservation in general, in specific I think that there's a forest-trees situation here. Having the kind of tight house they're aiming for is a bad in itself. An air-leaky house will likely not have backdrafting problems or develop vacuum conditions. It also means that you're not relying on a high tech system for your fresh air, even if you keep all the doors and windows shut. It means that you have a fighting chance against potentially dangerous chemicals that may get into your air. It means you can smell the wildfire before your house starts burning. It's much more in tune with the earth. Of course, it's an energy suck, which is why they're fighting it, but at some point there has to be a balance. There is a happy medium where you don't have cold drafts rippling around your ankles in the Winter, but aren't buttoned up so tight that your hood can deprive you of oxygen and suck the ashes out of your fireplace.

It's true that if you're heating or cooling your home, you'll have to heat or cool the same volume of air as you're exhausting with your hood. But is that really a reason not to have a working hood? I don't think so. And if you add the energy (and water, waste water treatment, and cleaning supplies) spent on laundry, cabinet cleaning, window washing, and all the other consequences of not having a good exhaust fan, are you really saving that much? For all that they're wringing their hands over having to have a system to treat the make-up air (and one that likely cannot use passive energy flow to do its thing), I wonder if having a drafty house might not be more efficient than having a heated/cooled/dehumidified make-up air system. :)

What I liked about the article was it's plea for more information to be put out there for homeowners. When a salesperson recommends a powerful hood, or someone on the appliance forum throws out a causal "oh, you'll need 1200 cfm with that" somewhere the caveats should be mentioned: Such fans have the potential to cause dangerous backdrafting, so be careful in how your home's systems are set up. Many municipalities have code requiring make-up air above a certain cfm, so check first because it may cost many thousands extra to get it installed. And weigh the possibility that such a conditioned make-up air system might be a big energy user.

No one should have any of these factors be a surprise after they've put a hood in and are trying to pass inspection.

The article and MUA issues aside, what no one mentioned so far in calculating the necessary CFMs is the length of the duct to vent the hood to the outside. The further you have to go and the more turns you have to make, the larger number of CFMs you will need to vent efficiently. Also, as trailrunner mentioned, you are better off with the higher CFM hood running on low as it will be significantly more quiet. That's my 2 cents worth, other than that I'm in no position to comment as my hood is still not installed.

This is OT and I don't want to hijack more than I already have, but I will say that we had a blower test + infrared scan of our house done as part of an energy audit earlier this fall, and it was probably the coolest house-related thing we have done---well worth the cost for everything we learned! They suck the air out of your house, which makes immediately clear where all the leaky spots are. Then they scan the wall to see where it's warm and where it's not, which tells you how well it's insulated. And, like plllog said, turns out a drafty house is not always bad, depending on what your goals are---we were deciding whether or not to add heat to our kitchen at the time, and based on the results (which showed that we were getting huge solar heating through a sw-facing wall), we followed their advice to skip the heat and *not* insulate that wall, thus allowing solar heating of the kitchen to continue. I've since learned that the house was probably oriented that way intentionally to allow sun to warm the kitchen in the summer; the original coal furnace was vented up through the kitchen and warmed the space in the winter months. Pretty cool! The blower test in particular could help tell you if you have much air coming into your kitchen independent of a MUA system, too.

I wanted to mention my deep committment to massive capture area. I have often stated that if I ever build my own home from dirt up I'd design my kitche ceiling in a large coved shape with a fan right at the top.

I would just do that over one quarter of the kitchen, where I would concentrate all the ovens toasters and smell makers. Over the cooktop is where the duct would connect to this container. Shaped like a valance or header box over the counter, it would all be one big hood or liner. Halogen lights in the front edge, and the rest being an open catchment containment capture volume space for extraction, or exhaust, with a fan (blower) motor somewhere remote halfway up the conduit.

To replace the air that has to leave the house when certain kinds of cooking take place, make-up air can be provided in the same way that it is provided in some fireplaces and chimneys: a direct inlet pipe from the great outdoors. Unconditioned air. Since this air gets directed onto the fire, it goes up the chimney. Same principle would apply to the air needed around the cooktop. Then I would wok in peace, during the coldest winters and hottest summers. MAy we all one day wok in peace.