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teresacooks

range hood duct size question

teresacooks
15 years ago

So in reading threads about range hoods I gather that a bigger duct size is better, I think I have room for that, need to start from scratch etc. . . Here's the thing, I'm looking at a futuro futuro and it says the duct size is 4" round on some and 5" round on some this is with 800 CFM. Am I missing something this is smaller than the 6" people are complaining about???

Comments (14)

  • paddy_99
    15 years ago

    Mine is 1200 CFM and it has a 10" duct. I think 800CFM would take at least a 8" duct.

  • davidro1
    15 years ago

    I have seen 5" duct before. Yes, the blower will work ever better, with a larger duct than spec'd. Faster startup and less air flow noise at startup and at steady state. Without studying this company's products, I'd say they must have decided long ago to target the market niche of people who have little space for ducting.

    A friend of mine put a length of 5" duct in his new kitchen. He has consulted Consumer Reports and ASHRAE documents, and yesterday he ordered a CR-recommended Broan product designed for a 6" duct and producing just under 300 CFM. Undersizing his duct for a few feet doesn't scare him since Broan has other products sending 500 CFM down a 6" duct. He is a mechanical engineer who has designed turbine blades for a living so he knows how to calculate air flow, air pressure, friction losses and much more. According to him, it's amazing how much air they can send down a duct these days, so he is comfortable undersizing his kitchen duct for a few feet.

    So sending 800 CFM down a 5" is quite a feat but doesn't sound impossible to me, thinking intuitively, not based on experience.

    Do you need 800 CFM? According to n AHRAE article, it all depends on "plume size" whether you need a lot more than the average 200 to 400 CFM kitchen air extraction. With 40000 BTU gas, if two burners are running hard it makes a plume much much smaller than all four burners on High; then you would need to plan for more, like, say, 600 CFM and makeup air. It's insane to plan for an air flow so ridiculously high that it pumps heat out of your house and even risks sucking the flame back out of your furnace.

    A 6" duct on a system designed for 4" or 5" duct will make it perform better.

    HTH
    -david

  • teresacooks
    Original Author
    15 years ago

    Hmm,
    well basically I just like the look of the hood. . . I will have the duct taking a turn (venting up then out the side of the house in a soffit) so I think that might require little extra power, I"ll have a fratelli range, so not superpowered, would I just basically use the extractor on low most of the time??

  • davidro1
    15 years ago

    sounds good. It's a short distance. Enjoy it. It'll be on low most of the time, nice and quiet. Don't let me overthink this.
    -d

  • lascatx
    15 years ago

    You can move a lot of air through a 1-1/2 or 2" vaccum hose, but your motor has to work harder and it's probably going to be noisier -- both from a harder working motor and more air moving through less space.

    I have a 1200 cfm hood with a 10" duct also. It is quieter than my old downdraft with about a 5" duct and who knows what cfm rating (I'd be surprised if it was more than 3-400 cfm).

    Other factors are the motor and the overall build. I looked at some Futuro vents and thought they were nice looking. There were a few people here who had them and were quite happy with them at the time. Look at things like cfm ratings, duct size and such as general comparisons, but also look to see what folks who actually have a brand are saying about them.

  • danielle00
    15 years ago

    I've never heard of needing makeup air for 600 cfms.. anyone else with 600 cfms have makeup air? just wondering since we'll have more than 600 cfms in the new kitchen.

  • boysrus2
    15 years ago

    I have a Best by Broan 1200 cfm, 10" duct, external blower, and I didn't do make-up air. Rarely is it on at full tilt, and I've not had any issues. It's fairly quiet too.

  • morton5
    15 years ago

    I have a 48" Futuro plane hood. The spec is for a 6" duct, and that is what I have installed. I am very happy with the hood. It is powerful, quiet, and easy to clean. The capture area is admittedly poorer than a chimney-style hood, but if I turn the blower to high and get a good draft going before I sear or stir fry, I have no issues.

  • cooksnsews
    15 years ago

    Re make-up air... It all depends on where you live and the local building codes. Where I live, any venting system 300 cfm or greater requires make-up air. My 600 cfm unit required heated make-up air. The infrastructure and installation costs were approx 4X the retail cost of the fan + hood.

  • mamadadapaige
    15 years ago

    just wanted to throw something else out there to consider. We had to add a backdraft damper to our duct work. there was a massive amount of cold air rushing in. The GC's hadn't included the damper to start with because they said they had heard from other clients that it interfered with the functionality of the hood (not sure this is true).

    Anyway, it was a bit of a pain to add it later so just wanted you to consider this up front.

  • sweeby
    15 years ago

    "I've never heard of needing makeup air for 600 cfms.. anyone else with 600 cfms have makeup air? just wondering since we'll have more than 600 cfms in the new kitchen."

    Whenever air is pumped out, the same volume of air needs to come back in -- 'make up air'. In my kitchen, there's a window about 10 feet away opposite the cooktop and 600 CFM vent, and whenever I run the vent on anything above 'low', I crack open the window. The difference in performance with the window open is amazing! With my vent on high and the window open a bit, I can blacken redfish on an iron grilltop and not even have the kitchen smell like fish.

    Duct design is almost as important as duct sizing. For example, every twist and turn reduces ducting efficiency, requiring the fan motor to work harder and causing more air noise. Same with shifting from round duct to rectangular or vice versa. But shape-shifting at the same time as a turn basically gets you only one reduction in efficiency rather than two. And two 45 degree turns are more efficient than one 90.

    There was a fabulous duct-design worksheet online (my link no longer works, but worth searching for if you're in the process) that showed the efficiency loss per linear foot of ducting for each type of duct piece. It also discussed duct sizing (If I recall, too big wasn't good), and had a calculation sheet to help you determine how many CFMs you need.

  • cooksnsews
    15 years ago

    Opening a window is not always a possibility. Mine has been frozen shut since mid-December. And even if I could break the seal, I'm not sure I want a blast of winter air up my backside while I'm cooking.

    Mind you, I'm wondering if my venting performance will improve come summer, when I can open my kitchen windows. My new kitchen wasn't completed until after freeze-up.

  • charlessmith44_gmail_com
    12 years ago

    I am trying to buy a 36 in chiminey hood for a kitchen I am remodeling. The cooktop is 30 in and electric, our kitchen has an 8 ft ceiling. Low noise along with performance are very important factors. I found a couple hoods; Cavaliere and Kobie. They are both under $1000 but I am concerned that they have a poor design. The hood design uses 6 in duct for aprox 700 cfm. I contacted the companies and they strongly urged using 6 in duct all the way out. They said increasing duct size to 8 in would possibly make particles not be removed properly due to lower air velocities, especially at fittings which I will have 2 45 degree bends, I will also have aprox 15 ft of str. duct. I feel they have another reason for keeping the duct down to 6 in. but not saying. I Know that trying to push 700 cfm through 6 in. duct in HVAC ductwork does not work. I would like to keep the noise level down to 2 songs at 300 cfm if that is possible. I guess I want to get the quietest unit possible and remove all the air entrained particles possible. I know noise is generated from mechanical as well as air movement but dont know how much I will gain by installing to 8 in. duct and if that will, in the long run, hurt the hood performance. Is any place on line that rates hoods by noise levels. I would greatly appreciate any info on my problem Thankyou