So how man CFM's do I really need?

12 years ago

We are remodeling a kitchen and am thinking of putting up a wall mount hood instead of the OTR microwave hood. I am probably going with Kitchenaid, or Frigidaire slide in gas range (nothing real high end). I don't cook that much, several times a week, but not every day.

We are also on a bit of a budget, so I'm wondering what kind of range hood would be sufficient for my needs that would look good without breaking the bank?

Thanks! BTW seems like a lot of threads about range hoods lately.

Featured Answer

davidro1

12 years ago

ditto weissman.

I'll modify this statement:
"... I can tell you anything under 400cfm is a waste no matter what you do otherwise..."

Here goes.

There are millions of cheap propellor blade fans installed, that function, in a sort of way, removing most of the smokey greasy air that some kinds of cooking generates. They do work. They are rated at various CFM numbers all less than 400 cfm. In reality, these blower motors are very weak if the air in the duct meets up against resistance. So, a short duct through the wall = they work OK, while a long duct up through the attic to the roof = they work erratically or poorly, and sometimes well. Some people have never seen these "hoods". Cheap rental apartments have them. You can see them in cheap hardware stores.

The rated CFM is just a lie printed publicly which nobody can refute. It was the cfm produced in a resistance-less tube in a laboratory. In reality, something innocuous can make the fan perform poorly. Many things. Even something as dumb as "wind outside" i.e. your regular everyday weather.

---

The next thing up the scale of goodness is a squirrel cage blower. You can see them in cheap hardware stores too. Some are weak, some are strong. Some can push air very well no matter how much resistance the duct length causes. They are all "rated" for a CFM in the hundreds.

300 CFM is fine if you truly get 300 CFM and if your cooking style compensates when the hood needs a 1/2 minute to catch up.

Blowers inside "OTR" microwaves have to push air through tight spaces that bend and turn before the air even gets out of the microwave and into the house duct. I wonder if the blower motor inside an OTR microwave was "rated" only prior to it being installed inside the microwave. I suspect this is the case. Then, the OTR microwave can be sold as if it had a Large CFM while it might never ever reach that CFM under ANY circumstances. Let's get the Consumer Reports police to look into this.

Although it is true that air can move around anything, it is also completely false when you want SPEED. Obstacles and resistance put a maximum cap on speed (of air flow).

The chart they publish with the fan blower motor shows you how much degradation in performance was observed during the lab test, when a form of air resistance was created. But this too can lie, in a way, even while it remains true and accurate.

Your duct diameter or duct dimensions are big factor. When you post these dimensions you will get more specific advice.

Hth.

Here is a link that might be useful: help make a hood decision (kitchen forum)

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Comments (10)

asolo
12 years ago
Has to do with combination of capture area as well as CFM but I can tell you anything under 400cfm is a waste no matter what you do otherwise.

Otherwise, suggest variable speed control and fan mounted outside to reduce noise.
JPRain
12 years ago
I would search Craigs List and Ebay for a good deal. That will save you money more than anything.

Also, make sure you know what the up air requirements are in your area. I know some towns in my area require a make up air system if you go over 600. MUA will ad cost. If you're pulling a permit they might check.
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Start with the uprising effluent velocity of around 3 ft/s or 180 ft/min, multiply by the hood aperture area in square feet. This is the air flow (cfm) required to capture and contain the cooking plume that is intercepted by the hood if the surface the rising plume meets is relatively flat and horizontal. With less velocity, some effluent may "reflect" (curl) out of the hood. For pyramidal hoods, usually sold with baffles, the interior reflection angle is predominantly upward, and this allows the cfm to be less because some of the effluent momentum is still carried upward after the first bounce. One might get away with a factor of 0.5 on the product of aperture and plume speed. For example, a 6 square foot baffled hood of the pyramidal style (e.g., commercial, Independent, or ModernAire baffled hoods), would require around 540 cfm (actual). Please consider that due to various duct pressure losses, hood pressure losses, and house negative pressure, the actual flow rate that a particular blower can move will be significantly less than its rated (at zero static pressure) flow rate. So, a four square foot hood would require 2/3 of the above calculated value, or 360 cfm. But to make up for the losses, the blower should be rated perhaps as much as 1.5 times larger to actually move 360 cfm. C.f. Blue Star's choice of words: "... move a minimum of 300 cfm." Please consider all these values nominal, and dependent not only on many factors not accounted for here and rarely specified so that they can be accounted for, but also on user tolerance for odor, type of cooking, etc. kas
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Currently I have no ventilation. I don't even have a stove pipe, so we sometimes have a lovely "gas" smell in the kitchen. I do little frying/sauteeing b/c the wall behind the stove gets so gross! So I'm looking forward to doing more! But only on one burner at a time. And the griddle is strictly for pancakes (so no fat frying). The stove is 39", so I'll need the 42" hood. Eventually I'll have a cabinet unit on one side only. I *think* the 600 would be between $200-400 more (I was quoted 1471 for 400cfm at 18" high vs 1855 for 600cfm and 30" high. So those prices encompass 2 different heights as well as blowers. And rats: just realized those prices probably don't include those nifty cute bands that give the hood its retro look. Oh shoot!!!! Why does everything stylish have to be soooooooo expensive!!!!!???????TheHusband and I are not $2,000-hood-people. Rats, rats, rats!!!!!!! The Modern Aire is just much more beautiful than the Vent-a-hood. Francy
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Ccat - I think you will find the link below helpful; it is the Appliances Forum FAQ on Vent Hoods. You mention raising your hood high above the range - that could be a major mistake depending on how high you are talking about. You will diminish and dilute the exhaust power substantially if the hood is too high. The hood should come with specific instructions about the required distance from the top of the burners to the bottom of the hood (usually these specs are posted on line). There have been threads on this forum of people either having to lower their hoods after-the-fact or being unhappy with their hoods cause they are too high above the burners and have lost effectiveness, despite having plenty of cfm's. As to the amount of cfm's needed, that is explained in the attached link, but briefly, "10CFM per 1000BTU of burner capacity. Thus, for a 60K-BTU cooktop (4 burners x 15K btu each), you should have a 600CFM hood. Since you rarely, if ever, have all burners on full at once, you have excess capacity that can be used when you really need it." Here is a link that might be useful: Appliance Forum FAQ on Vent Hoods
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The hood's width at 6" wider than the cooktop is ideal, but many people have the hood the same width as the cooktop, and it's fine. Often people don't want to give up the 6" of upper cabinets, or cannot, because the cabinets are already there. If you have the space for a 42" wide hood, it would be the better choice, but is not an absolute requirement. The thinking on hoods has moved somewhat from emphasizing the width of 6" wider, to more emphasis on the depth of the hood front-to-back. Many hoods sold these days are only 20" front-to-back, leaving the front burners uncovered. That is more of an issue than ensuring extra width on the hood. Find a hood that is close to 24" front-to-back if you can. The height of the hood is ideally 30-36" between the top of the cooktop and the bottom of the hood. If you install higher than that, you are allowing too much space for smoke and grease to escape and float around the kitchen. Go lower than that and your cooking area will feel cramped. I want to reiterate what Stan Z said about the correct ducting. You can have the best hood in the universe, but if your duct diameter is too small, your hood performance will be constrained. Most hoods require 8" or 10" diameter - the specs will say which. You can find hoods with specs for 6" diameter, but those hoods will be noisy, as air is pulled through a more narrow space. 600 cfms is usually enough for most people with a 36" width cooktop. Underpowering the hood is a mistake - you will have spent all that money on the hood, and it will be inadequate from the get-go. If you are thinking about going with more than 600 cfms, the advantage would be that you can run the hood on a lower speed setting most of the time, which will be quieter. The disadvantage of more than 600 cfms is that may require make-up air depending on your locality, building code, and how tight your house is.
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Nunyabiz1
12 years ago
Not sure what your budget is but ours was $2300 total for stove and OTR microwave which works fine as a vent. I do cook everyday, usually twice a day but just for 2 people.
We went with a NXR stove and LG microwave that has 400CFM.

After installation and everything we went $100 over budget.

Unless you are cooking like a madman everyday using all 4 burners and the oven at full blast like you are in the Iron Chef kitchen then you do not really "need" some high CFM hood.
In my 35 years of cooking I have never had a hood yet.

Now if I won the lottery and I built my dream house with my dream kitchen which would be a VERY large kitchen complete with a 36" 6 burner gas stove with a 36" professional flat top griddle sitting next to it then yes I would have 2- 48" hoods side by side over the top 1000CFM each outside motors and hanging over several inches.
Even then I wouldn't "need" it because I would have high ceilings and great ventilation throughout the house anyway but I would "want" it.

But now in my tiny kitchen with my 30" 4 burner stove which still puts out a respectable 60,000 BTUs if all 4 burners are going, a decent OTR microwave with 400CFM works just fine, especially since I would have no other place for the microwave if it didn't go over the stove.

Just depends on what your definition of need is.

Next time I cook some burgers on my stove top griddle with 2 burners going on 4 burgers and buns and onions etc, which generates probably about as much smoke & grease as I ever produce, I will try to take a picture from front and side to see if I can show if any smoke is getting by the 400CFM OTR microwave.

Here is a link that might be useful: Range Hood Ventilation
davidro1
12 years ago
at fantech.net you will find small inline blowers. This is good for you on a budget. Blowers from Fantech are squirrel cage blowers, not fans with propeller blades. Any blower from Fantech will be a bigger blower than most of the inexpensive exhaust hoods that you might see at a hardware store or in a rental apartment. Note that the claimed CFM is only a "rated' number based on a lab test that has no obstacles. How it performs when the duct is bendy is never discussed.
Mercymygft
Original Author
12 years ago
Nunyabiz1... hahaha! Your post made me laugh!

On a side note... over on the Kitchens forum someone was asking about putting two 36" ranges side by side in a kitchen. I'm thinkin' holy smokes!! if you need two 36" ranges you are doing WAY TOO MUCH COOKING!!! Or your last name is Dugger ;-)
jgopp
12 years ago
I have a 650CFM and it works very well. Never had a problem it captures everything very fast even on a lower setting. Not sure why people seem to think they need like 1200 unless you're running a 48 pro range and searing steaks and what not on it.
weissman
12 years ago
Unless you have a grill or a very large range, 600 CFM will be more than enough and won't require make-up air in most locales. If you have the option for a real vented hood, an OTR is a bad idea, not just because of the low CFMs but because of the poor capture area.
aprince
12 years ago
600 CFM is usually fine over a gas cooking surface. A couple other things to consider:
-You don't mention if quietness is important to you. The larger the CFM output on the blower, the quieter it will operate on low, the most common setting. Also, 6 inch ducts tend to make more noise than 7, 8 or 10. Utilize the duct size that is recommended on the hood you purchase.

-You also said you were going with a wall mount hood instead of an OTR microwave. Does this mean you are going to knock out cabinets to place the hood or will you still have a cabinet. I find that you get more bang for your buck if you go with an under cabinet unit instead of a wall mount unit.
Mercymygft
Original Author
12 years ago
Thanks everyone for their input.

aprince...Noise is really not that big a deal for me considering the amount and type of cooking I do. So if it's a little noisy... eh.

I am considering replacing the cabinet OTR with a wall mount because I think it would look cool, be a focal point, and I store junk in my OTR cabinet now, so it's not like I'm losing storage. Plus what I save on the price of the cabinet I could invest in the range hood. Another thing is I am really short so I can barely reach it anyway.

So how man CFM's do I really need?

davidro1

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So how man CFM's do I really need?

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