I don't to this for a living, but my family was in the HVAC business, some years ago.

A good rule of thumb is 100 CFM for every 10,000 BTU's. So if your range is going to put out 80,000 BTU - then you need an 800 CFM fan.

The problem is the Make Up Air. If your home is 'tight' - and you try to exhaust 800 cfm of air - you may wind up pulling air from combustion sources (like furnance / water heater / fireplace). The makeup air can be the expensive component.

I'm in the same boat, w/36in induction. Need answers and wanted to correct what I think is one mistake in doug-gb's post. I heard it is 100cfm per 1,000 btu, not 10,000 btu. Many gas tops, for instance, land around 15,000 btu of heat, and therefore call for the larger 1,000-1,600 cfm hoods, etc, etc.. We were headed for gas and switched to induction at the last moment, which from reading is telling me 600 cfm ought to do. Still curious which liner to put in custom built hood. thx.

Just finished my hood and I hate to tell you - $800 won't do a high end system (or even close). The Fantech inline system I installed was $800 just for the fan, silencer, roof cap, duct, etc. (1200 cfm) That was with me doing all the work & buying from the internet. That was before dropping major coin on the Modernaire vent hood.

what climate are you in?

do without a silencer.

go with a lower CFM. My experience with Fantech inline blowers was low cost.

Any upside down trough shape will do fine.

Building your own everything means you can meet your budget.

Many people get way more CFMs than they need. For most applications 600 CFMs is sufficient. If you have a grill on your cooktop then you will need more. One good thing about 600 CFMs is that most locales do not require make-up air with 600 CFMs although some northern states do require make-up air with anything over 300 CFMs. Hood manufacturers are notorious for trying to sell you more CFMs than you need. On the other hand, getting a deeper hood with larger capture area is always a good idea.

Ponder these questions:
1. How brilliant do you want the lighting?
2. Overall venting capability? Good? Better? Best? Or just a piece of artwork in the kitchen?
3. Noise level? As Weedmeister alluded too, the bigger the blower, the quieter the operation under normal circumstances.

Hoods perform best when slightly larger than the cooking area. Heat, odor, grease, steam and smoke don't rise straight up.
600-800 CFM should be fine. I recommend finding one that uses an 8 inch duct. This will be significantly quieter than 6 or 7 inches Your owners manual for your cooktop will have recommendations on the venting requirements, i.e. CFM and hood height.

An in-line blower, installed correctly, will usually be quieter than a roof-mount but the difference is not substantial.

To clarify the fuzzy math in other posts. CFM recommendations are calculated by adding all the BTU's on a cooktop and dividing by 100. A 60,000 Total BTU cooktop should be vented by a 600 CFM capable hood.

Induction does not utilize this formula.

I am spending under 800 - it seems like venting can get out of control expensive, and I have a hard time understanding the value of a mega powerful hood.

I agree that it seems like hood manufacturers try to sell you more cfms than you need. The rule of thumb calculation mentioned above assumes that you have all burners going at full blast (which, honestly, will never happen.). I am getting a 36 capital precision with 6 burners. Each burner is 19,000 btu. I still think I will be fine with a 600 cfm hood. Am I making a big mistake??

Right now in my soon to be demolished kitchen I have NO hood over an old Kenmore. Its not a good setup, but I figure going from 0 to 600 cfms will be a big improvement.

There are many ways to collect evidence to make a strong claim that you could get by with a 450 CFM blower. This is what many European companies sell. Nobody has ever said that Europeans have greasy kitchens. That is one strong argument.

Designnov - although it is unlikely that all burners will be going at full blast, keep in mind noise levels. Noise levels seem to be the No. 1 complaint about hoods. If you size your hood to accomodate the maximum btu's, that means most of the time you can use your hood on its lower setting, which will be quieter. Conversely, if you undersize your btu's, you will be using your hood at its utmost capacity even if you are just using 1 or 2 burners, and you will not like the noise level. At the same time, on the rare occasions when you need full capacity, like frying fish, or using those Capital burners to get a full char on a steak, you will happy to have the most cfm's you can. I mean, what's the point of buying a range like the Capital if you aren't going to be able to char a steak without setting off the smoke alarm? I would at least go to 900 cfm's if I were you.

I also hope everyone reading this thread understands they will need to have the proper size duct from the hood to out of the house. Most hoods need an 8" or 10" duct. If you connect them to a 6" duct, you will get really a lot of noise from pushing air through too small of a space, and you will get much less efficiency. Duct size is usually the reason why we hear people with identical hoods have different experiences with noise and efficiency.

Davidro - I had to chuckle at your statement about European kitchens. Where do you get the info for your "strong argument"? Have you done a poll of European kitchens? Have you visited a large number of European kitchens and done a statistical sampling? Do you have evidence that European kitchens are less greasy than American kitchens?

As to whether one can get by with 450 cfm blower as Davidro said, of course one can. Whether a family wants to "get by" or whether they want good exhaust depends on what kind of burner output they have, and what kind of cooking they do. If you're a vegetarian family, and don't stir fry, then 450 cfms will be great. If you mostly eat out, or bring in from the deli, then you don't even need 450. So you can't really make a generality like that.

I don't understand why vegetarians require less venting than us carnivores. Are veggie soups less steamy than meat stocks? Do vegetarians never burn anything? The smokiest kitchen situation I ever instigated concerned overcooked toast.

There are many ways to collect evidence to make a strong claim that you could get by with a 450 CFM blower.

This is What European Manufacturers sell. Go See Their Web Sites.

Nobody has ever said that Europeans have greasy kitchens. That is one strong argument.

Cooksnews - Vegetarian kitchens might need less cfm's for venting because meat gives off a lot of grease. The exhaust hood is not just for smoke and steam, but also for grease. If you have an effective hood, and you go to clean the baffles, you will see just how much grease can collect if you are cooking meats. As you pointed out, a vegetarian kitchen still will have burnt things and smells, and if fish is being cooked or fried, there will be smoke and grease. It seems that people in this thread are looking for reasons to pare down their cfms, to argue for minimal cfms. So I made the point that if your family is vegetarian, that might be one argument that you'd need less cfm's than a meat-eating family. Actually, I don't know why everyone is so keen to lower their cfms. They are paying a lot for a renovated kitchen, for appliances, tiling, counters, etc. but for some reason they are eager to cut back on the utility of their hoods. The only solid reason I can think of to undersize your cfms is, as Weissman pointed out, the need for makeup air because your house is tight or because of local codes, that would make it expensive and difficult to have higher cfms.

I agree with the above, about "less CFM's for vegetarians" because it illustrates the inevitable fact that people end up cooking whatever their house can handle, and they avoid cooking things that their house cannot handle.

In order to avoid causing problems for yourself, it is always good to look at the opposite side of anything you are considering. It helps you figure out what your tradeoffs are.

Sometimes things are overkill and that is OK, because it's just a little overkill. Sometimes you have to know when to stop, or when to decline to indulge in the overkill.

I could make a case for 300 CFM. First, I'll let it be firmly established that 450 CFM is a fine fine fine system, that is deemed adequate in civilized parts of the world.

I could make a case for 300 CFM. It will take more effort, more time, more battling others' resistance. That case might have to wait.

Looking at the opposite side of anything you are considering helps you figure out what your tradeoffs are. It helps you avoid being a lemming!

Previous discussions in this forum have admitted that people CAN indeed make do without any exhaust venting at all. This is not my position, but I can agree when someone explains why they will choose not to install any exhaust venting. Let's all be openminded here. The OP has some skills and some money, and is not looking for the best or the ultimate. The OP desires to figure out what tradeoffs are. Let's not ridicule any position.

A 450 CFM blower can work well. There are millions of 300 CFM systems installed in rental apartments. Why was 300 CFM deemed to be the right tradeoff for renters? I think it was because of the long long 6" duct that was designed into many apartment buildings.

The OP should plan for a bigger duct than 6". My opinion.

This is a very interesting and informative thread. I confess not to be much of an expert on ventilation. However, I will say that I have been a forum member for several years, and I cannot recall even one poster coming back after installing a hood to complain that they have too many cfms. There have, however, been a good number of threads about people complaining of inadequate cfms. Some of the complainst may have to do with too-narrow ducting, as has been mentioned. People do tend to spend a lot of money on powerful professional-style ranges, and then paradoxically want to install a hood with as low cfms as they can get away with, and that doesn't seem logical.

shannonplus2, I agree that it does not make much sense to buy an expensive range and cheap out on the hood. Im just trying to figure out if the rule of thumb for calculating cfs is overkill. Why buy a hood based on a scenario that will likely never happen? (using all burners at max heat). I have never owned a hood that vents outside, and while I definitely see the need for one, it also doesnt seem logical to spend twice as much if I wont really notice much difference. Just trying to feel members out to see if this theory makes sense.

Id love to read the posts about members complaining about inadequate cfms. I will do a search.

As I mentioned above, one reason for not putting in too many CFMs is the need to put in a make up air system which can be very expensive.

ditto weissman about the air, that has to come from somewhere!

ditto akchicago about the duct size.

Make up air is jurisdictional - I'd call the zoning people & ask, I don't live in a zoned area so it doesn't matter what I do. I just crack a window for MUA. That Precision will put out some heat so more CFMs are better. Whatever you decide to do, make sure you plan ahead in case you need more.

As far as Europeans getting by with less - have you been to Europe & seen std European kitchens? I have - there's not enough room for the duct work to be large enough to handle any more CFM

Designnov - is it really, as you said, "twice as much" to go from the 600 cfm hood you want, to a 900 cfm or 1200 cfm hood that may perform better/ be less noisy? I honestly don't know the price differential, but I don't think it is linear to the cfms, e.g. that a 1200 cfm hood would be twice as expensive as a 600 cfm hood. A lot of the cost of a hood is in the canopy, i.e. the metal and fabrication, which would be the same even with different cfms (assuming you are choosing the same hood dimensions, but looking at different cfm's offered for that hood) . I would be very interested if you would report back on price differences between a 600 cfm, 900 cfm and 1200 cfm of the same hood brand and design. As I said, this thread is very educational.

Also, if the hood you are looking at is too expensive, what about going to another brand that might be less? Perhaps there are others that would give more bang for the buck. You could then get the appropriate cfms without wondering if you are shortchanging yourself on hood capability, yet stay within your budget. What hood brands are you looking at?

Hi - this thread is a little old now so this might be too late, but I have some related Qs even after reading all this helpful advice. I'm starting gut reno of a 120+ year old townhouse in NYC and plan to install 36" bluestar with 2, 22K BTU burners (in addition to other smaller burners of course) which we'll use primarily for stir fry. We HATE grease smell in the house and cook all the time, so want to get what we need for the stove we have. So I'm fine getting major CFM, if that's what it takes, but my concern is the ducting. It seems we have 2 options - up an existing chimney (which we're having relined and is not used for fireplaces; fireplaces were all sealed up decades ago) - but that would be 3 stories or about 50 feet needed to get to the roof and I wonder if that's too far for it to function properly...2nd option would be to bring it down into a soffit on the floor below (a rental apartment that already has a dropped ceiling) and vent out of the exterior wall, but this would require making a new hole in our masonry, and would mean the venting would be just outside our kitchen, under a french door that we're likely to want to open often while we're cooking - so the smoke could in theory end up back in the kitchen, no?

I am by no means an expert so forgive me if I'm totally off base here, but any thoughts/advice would be much appreciated. Thanks!

If these are your only 2 options I'd go with the existing flue providing that the hood is the only thing operating in the flue.
You said that the fireplaces were sealed up years ago but what about your heating plant.
It's probably agianst code to vent into the heating plant flue.
If you're going to use the flue, I'd go with a 1200CFM minimum and an external blower mounted at the top end of the duct run.
A hood this size will require at least a 10" duct so make sure the flue can accommodate a duct of this size.
Also why are you lining the flue?
If you're going to run the vent hood duct work through this flue then the lining would seem unnecessary.

OOOOOPS!

The above post is meant for empathie.

Thanks very much, willtv. Can i just clarify, though, if when you say "heating plant," do you mean the flue that the boiler vents into? b/c if so, that's a different one on the other side of the house.

why are we lining the flue, well, b/c that's what we were told to do. bricks from the top have been falling through the insides. if we're not going to use the fireplaces and we fix the chimneys at the roof, do you think we'd be okay not lining, then?

thx again!
e

empathie, Yes, I was referring to the boiler.
So then I assume that the vent hood will be using a former fireplace flue.
As for lining the flue, I'm no expert but, it seems to me that if the bricks are falling in from above the roof line, they need to be re-pointed even if you line the flue. If the bricks below the roof line are sufficiently degraded that they won't support the weight of the chimney, then relining is definetly required.
Just be sure that after all is said and done that you'll be able to fit a 10" or 12" duct in the flue because that's what your vent hood will require.

hi willtv - thx so much, again. need to have a chimney expert out to see exactly where the trouble is, but given everything else we've found so far on this house, am guessing it's likely to be the more complicated option :) (weak bricks both above and below roof line)

point taken about the size of duct we'll need - it's a big FP chimney so hopefully even with liner there will still be room. But i'll be sure to raise that with the chimney repair folks.

thanks again!

empathie BTW you're going to love that Bluestar.
I installed one in our recent kitchen gut reno.
Here's a shot.

It is, as some on this forum refer to it, a box of fire.

I wonder if it is more cost effective to remove the failing chimney and run a duct up the former path of the chimney.

A smooth metal duct will capture less grease than a chimney liner, I would guess, if the liner is tile.

Outside the duct below the attic one might need (especially for NYC) a chase with fire code sheetrock on each side. Metal studding of some sort could keep the chase thickness low. All of this might well take no more space than the chimney did.

However, if an inline fan is used and/or a silencer is used, and these are within the living quarters, the chase would have to be larger for some distance.

The choice of where to put the fan will depend on the house configuration. If the duct is 10-inch as usually used for large residential hood ventilation, and 50 feet long, care in fan selection is needed to ensure sufficient flow for the pressure loss of duct, hood transitions, baffles, negative pressure in the room due to insufficient make up air, etc.

kas

I recently had to figure this out as well. I had a friend get on the roof while I cooked something over the stove to test it. We then tested his house because he has a metal duck system and we had very different results. When I get home I will post the results.

Here is a link that might be useful: Appliance Repair San Antonio