how many cfm's do i need?

plumberryJune 24, 2010

Friends,

I am planning to get the Wolf 36" 5 burner gas cooktop (CT36). One store told me I need a hood w/~800cfm another told me 440cfm? Which one is right? or, are they both wrong? I'm looking at Imperial or Broan.

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weissman

600 CFM

    Bookmark   June 24, 2010 at 1:18AM
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ak0402

I agree with Weissman. To arrive at the 600 cfm number, note that the rule of thumb is 100 cfm's for every 1000 btu's. The btu's of the Wolf CT36G/S cooktop add up to 54,600. You won't find hoods with 546 cfms, so rounded up a bit, the cooktop will require 600 cfms. You may think that you'll not use all 5 burners at once, so why get 600 cfms. Well, it's better, and quieter, to be able to use the hood at a lower setting most of the time, and occasionally use all 600 cfms, than it would be to buy a 400 cfm hood and have to use it at full strength all the time, and still find it inadequate at times.

    Bookmark   June 24, 2010 at 10:12AM
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fun2cook

I'm looking at the same cooktop (Blue Star lovers, please don't hate me). For the hood Im looking at the Zephyr-Milano-G with 715 cfm or the Electrolux Glass canopy with 600 cfm. Both are 42" to increase coverage over my pennisula set cooktop. Is 715 cfm really necessary for this cooktop? The Electrolux is 2/3'3 the price of the Zephyr. Is it worth the extra? cost to get more cfm? Thank you

    Bookmark   June 24, 2010 at 10:56AM
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plumberry

fun2cook, akchicago explains that you'd have a 600cfm on low if all 5 burners would be in use. i like the idea of keeping the hood on low. they can get loud! I only have a 4 burner now and rarely have used all 4 at the same time. personally, i'm going to go with 600.

    Bookmark   June 24, 2010 at 11:23AM
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ak0402

akchicago explains that you'd have a 600cfm on low if all 5 burners would be in use

No, I didn't say that. All 5 burners in use at once would need the full 600 cfm strength. However, most of the time, most people don't use all 5 burners at once. For the most part, when you won't be using all 5 burners at once, you can have a 600cfm hood at the quieter, lower setting. For those times when you will be using all 5 burners, or perhaps be frying something smoky, you'll have the flexibility of being able to go to the higher full-600cfm setting.

    Bookmark   June 24, 2010 at 11:56AM
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amcook

akchicago,
I think the actual rule of thumb is 1 to 100 or 10cfm per 1k BTU. Probably just a typo since the rest or your math is right.

    Bookmark   June 24, 2010 at 1:33PM
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aprince

Is it worth the extra? cost to get more cfm? Thank you

The extra 115 CFM isn't going to make a big difference in extraction but as AKchicago sort of mentioned, a larger blower/motor will operate quieter on the lower settings.
These should have at least an 8 inch duct. If smaller, you will hear it more

    Bookmark   June 24, 2010 at 1:48PM
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weissman

The only risk with more CFMs is the need for makeup air. With 600 CFMs you usually don't need makeup air except in some northern states that require makeup air with anything over 300 CFMs.

    Bookmark   June 24, 2010 at 3:02PM
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jakvis

If you have a longer vent run you may need to use more than 600CMF to make up for the extra resistance of the longer pipe and elbows.
This is not really such a simple problem that you can say I have X cooktop so I automatically need Y blower.
Pipe size - 6", 8", 10" 12", Pipe shape - Round, Oval, Rectangle, Length of run, number of elbows, Pipe cap, all have to be considered.

    Bookmark   June 24, 2010 at 8:18PM
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amcook

I think 300cfm is just too small. And there's really not much between 300 and 600 so go for the 600. The cost different won't be nearly as much as if you get the 300 and a year later decide you really needed 600. Honestly there are so many factors such as the type of cooking you do and the location of the range/hood that it's almost impossible for us to tell you what's right. Certainly the sales person isn't going to take the time to give you the best information. IMHO, assuming standard wall mount with surrounding cabinets, then 600cfm would be enough for almost all types of cooking you might do. If you don't saute or stir fry much, then 300cfm might be enough. If you have an island configuration, I'd go one step up: 600cfm minimum and 900cfm if you stirfry or saute regularly. I had an island with a 900-1200 hood and when I'd do heavy duty stir-fry, it would set off the smoke alarm on occasion. That said, my config was not ideal. The duct was not a full 8" duct due to an odd bend in the attic. That meant I was not at full efficiency and it was noisy. I was also doing more stir frying than most (6 dishes in 45 minutes of non stop cooking). In general, 900 should be fine for an island install.

    Bookmark   June 24, 2010 at 9:09PM
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plumberry

This is all very interesting to me. I'm so grateful to this forum for the patient help you are providing. Thanks!

    Bookmark   June 24, 2010 at 9:41PM
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monkeymo

Heed weissman's post... check your local codes first. In my area anything over 600cfm requires a makeup air system. From what I have researched it can be quite costly to install considering in the mid to northern areas it would be required to be heated as well.

    Bookmark   June 24, 2010 at 10:17PM
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kaseki

One way to look at required flow rate (cfm) is to match the total effluent plume cfm, a value that might be estimated from published measurements. However, when one considers the two hood functions of capture and containment, one has to also ensure that containment is successful. Success requires the velocity of the air flow at the filters or baffles to be at least equal to that of the rising effluent, or the effluent may curl around and escape the hood. Hence, even if only one burner is on, the flow rate may have to be higher than that particular btu level's rule of thumb will suggest. In other words, all the flow doesn't magically end up over the operating burner.

Another factor to consider is that if the rule applies to the flow at the hood, as it normally would, then the fan has to be sized significantly larger for most ducting and hood designs. This is due to pressure losses at all the transitions, pressure losses due to friction in the ducting, and negative internal house pressure. Fans lose performance with pressure drop (pressure loss). If you believe you need 600 cfm, than a 900 cfm rated fan (at zero inches of water) may be required depending on duct design.

kas

    Bookmark   June 27, 2010 at 12:26PM
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fun2cook

Okay I'm still confused on cfm's versus duct size. My ducting is being replaced, so I can go with the size my hood requires, rather than match a hood to my current duct. The Electolux has 600 cfm with 8" duct. The Zephyr has 715 cfm with 6" duct. So is less cfm with wider duct better, or quieter? I live in So Cal where we can open the windows anytime. So I don't know about any code requirement for "make-up" air. Thanks

    Bookmark   June 27, 2010 at 1:00PM
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shannonplus2

IMHO, the wider 8" duct is better. It's just simple physics that when you push air through a narrower opening, it will be noisier than through a wider opening. Furthermore, there are more hoods on the market to choose from that specify an 8" duct. If you have a 6" duct, you are narrowing your hood choices a lot.

Get the 8" duct, since you are able to. I've been on this forum a long time, and I have never once seen a recommendation to choose a 6" duct over an 8" duct, but lots of recommendations to get an 8" duct, and also lots of posts from people bemoaning that they are stuck with a 6" duct due to pre-existing construction.

    Bookmark   June 27, 2010 at 1:17PM
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aprince

"The Electolux has 600 cfm with 8" duct. The Zephyr has 715 cfm with 6" duct. So is less cfm with wider duct better, or quieter?"

Better AND Quieter. 6 inches diameter duct is never enough over gas. Doesn't matter if it is 300 CFM's (not enough) or 900 CFM's. Think of air as being similar to water or electricity. Air will move to where you send it but if the duct is too small, the pressure will create a significant amount of noise. Just like water through a channel, the narrower the path, the more violent the water behaves.

    Bookmark   June 29, 2010 at 7:20PM
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amcook

In general, if you have a 36" or 48" cooktop/range, I'd suggest a 10" duct on new construction. Technically 8" is probably enough but adapting a hood that requires 8" to a 10" duct is pretty simple. The other way around will not produce the desired results. Trust me on this. I have first hand experience. A lot of this depends on the length of run but I would definitely not go below 8". For most home installs, 10" is the max you'd need. For instance, I'm planing a prestige hood with the RB1600 (1600CFM remote blower) and even that is spec'd at 10" duct. You'll only need 12" ducts if you are going 2000CFM system. BTW, the RB1200 is also 10" duct.

    Bookmark   June 29, 2010 at 7:59PM
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antss

I think a bit of info about how long you duct run is going to be, how many turns it'll have to make could be very important here.

If you have a long complex run 600 might not be enough.

    Bookmark   June 29, 2010 at 9:19PM
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fun2cook

Thank you everyone for your input. My duct will be a straight run, through the rafters. Probably about 8 feet long. The pre-existing duct was 8". My contractor says its very easy to install an adapter to go from 6" output on the hood to an 8" duct. So I guess that' the best way to go. You guys (and gals) are great.

    Bookmark   June 30, 2010 at 3:21PM
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kitchendetective

You may want to have a look at this thread and see if there is information that may apply in your instance.

Here is a link that might be useful: CFMs etc.

    Bookmark   June 30, 2010 at 5:48PM
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aprince

"""My contractor says its very easy to install an adapter to go from 6" output on the hood to an 8" duct. So I guess that' the best way to go."""

Easy to install? Yes but........

Ducting is only as large as it's smallest diameter. So if the output on the hood is 6", even if the duct is 8", the hood will perform like it's 6 inches.

IMO, 600 CFM with 8" duct would be better than 715CFM using a 6 Inch duct.

    Bookmark   July 1, 2010 at 1:48PM
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amcook

Larger ducts will reduce overall system resistance which is good. I'm not saying it will be night and day difference but certainly no worse and probably a bit better. You just need to make sure he does a good job with the adapter since bad installs where ducts form sharp bends can greatly reduce performance and increase noise. That said, I think the main reason is that it sounds like he already has 8" ducts so no reason to downgrade and if he ever wants to get a bigger hood/blower, the 8" duct is already in place.

    Bookmark   July 1, 2010 at 2:13PM
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dhs2005

Quick Answer: for gas range assume each burner produces 15,000 - 18,000 BTU. CFM is computed as 1% of BTU. Therefore a 5-burner gas requires then 750 - 900 cubic feet/minute (CFM) airflow.

Note that if the vent pathway between range hood and external exhaust has 45 or 90 degree turns then effective airflow is reduced and you will need to compensate for this in some way.

Anything less than 500 CFM is appropriate for some electric cook tops, inappropriate for any gas cook top.

Hope this helps,

Donald

Here is a link that might be useful: Vent4Less

    Bookmark   July 1, 2010 at 5:49PM
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fun2cook

Plumberry is the OP on this thread, and it's not my intention to take it over, but we share similar questions, and this information is so helpful to me.

"IMO, 600 CFM with 8" duct would be better than 715CFM using a 6 Inch duct."
This is the info I really was after. My 36" gas cooktop can produce a max of 55,000 btu. I have a straight run to my vent, (no turns) about 8 feet long. Current home ducting is 8*.
So I can go with an Electrolux/Broan: 600 CFM with an 8* blower, 42" x 20". Or it's the Zephyr, Milano-G: 715 CFM , 42" x 26" with a 6" bower, and then add an adapter to meet the 8" duct in my house.
Zephyr advantages are higher cfm, larger capture area, and lower sone rating.
Electrolux/Broan advantage is larger blower/duct. Surprising it's noise rating is higher than the Zephyr, even though it has bigger duct and lower cfm.
Features and styling are equivalent.

    Bookmark   July 1, 2010 at 9:26PM
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