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  1. #1
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    Jun 2011
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    kitchen exhaust duct size

    I am an HVAC designer but this is a question about an installation I am doing in my own house. I'm putting in a kitchen exhaust hood that is 480 CFM with a 6" round duct connection. I've never designed a kitchen exhaust system before but my understanding was that for bathroom exhaust you want to size the duct for 0.1" of head loss. My duct sizer tells me that would require a 10.5" duct. Using a 6" round duct as the manufacturer recommends will give me a head loss of 1.8" and a velocity of 2476 FPM. Is this right? Are kitchen exhaust ducts sized for higher velocities and head losses than bathroom exhaust?

  2. #2
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    Jun 2010
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    I'm putting in a kitchen exhaust hood that is 480 CFM with a 6" round duct connection.
    I've never designed a kitchen exhaust system before but my understanding was that for bathroom exhaust you want to size the duct for 0.1" of head loss. My duct sizer tells me that would require a 10.5" duct.
    AHH!!! the old rule of thumb that takes care of everything

    Using a 6" round duct as the manufacturer recommends will give me a head loss of 1.8" for 100 feet and a velocity of 2476 FPM. Is this right?

    Are kitchen exhaust ducts sized for higher velocities and head And short runs of duct
    In commercial there is a min velocity to carry the particulate away, of course the duct is also welded for liquid tight and insulated with fire resistant material so that when the grease is not cleaned like it should and catches fire, it won't burn your restaurant down
    I have often wondered about these Lt commercial kitchens going into homes and whether there should be a higher standard used.
    Sorry, I may have more questions than answers with this one
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  3. #3
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    Jun 2011
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    Quote Originally Posted by genduct View Post
    Sorry, I may have more questions than answers with this one
    Ask away, my friend. I am regretting not planning this out more ahead of time so I could have asked some of the guys at the office for advice before getting started (today). I think the answer to my original question is definitely "yes, the velocity should be higher". I still don't know if I should use 6" or 8" though because I have to make a lot of turns. The instructions for the hood says that if you have a long run you might want to use a larger duct size, but doesn't give specific recommendations.

    I think the full answer to this is that if the head loss on your duct run is sufficiently high (either a long run or with a lot of turns) you'd want to minimize the damage by using a larger duct size, which would have less head loss and ensure the static pressure at the end of the run is sufficiently high. I have a spreadsheet for this calculation and the ASHRAE manual with the losses for each fitting type at my office but a) I want to do this today, b) the manufacture does not give the external static pressure of the fan and c) I'm probably over thinking this.

    I'm boxed in a weird corner with how I need to run this ductwork because of the way the ceiling joists are above the fan so I have five 90 degree turns (1.0 diameter radius) over a distance of 23 feet. Do you think I should go to a larger size?

  4. #4
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    Quote Originally Posted by asdflkj View Post
    five 90 degree turns (1.0 diameter radius) over a distance of 23 feet. Do you think I should go to a larger size?
    correction: I have 6 90 degree turns, not 5

  5. #5
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    Jul 2004
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    Since the terminal vent of a kitchen exhaust is outside the occupied envelope, velocity at the termination isn't a real consideration. At the velocities you'll be running for a 480 CFM blower, you won't encounter any significant issues with a 6 inch. But if it makes you more comfortable, by all means increase it to 7 or even 8 inch but I wouldn't go above 8-inch. Bigger is not always better and static isn't a real consideration for a residential exhaust fan. Most of the noise encountered will be from the blower itself, not the duct.

  6. #6
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    Jun 2001
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    Moore, Oklahoma, United States
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    6 turns definitely will require an up-size in duct!! I'd do at least an 8". What static pressure is the 480CFM rated at?

  7. #7
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    Quote Originally Posted by 54regcab View Post
    6 turns definitely will require an up-size in duct!! I'd do at least an 8". What static pressure is the 480CFM rated at?
    It's not listed in any of the literature that came with the hood. I have not yet looked at the fan manufacturer to try to find the fan curve.

  8. #8
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    Jun 2001
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    Moore, Oklahoma, United States
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    Quote Originally Posted by asdflkj View Post
    It's not listed in any of the literature that came with the hood. I have not yet looked at the fan manufacturer to try to find the fan curve.
    If the manufacturers data doesn't have fan curve figures the 480CFM could be total BS. Size/speed of blower wheel and horsepower of motor might get you close to knowing real CFM. Odd they use a 6" duct connector for such a high CFM.

    You DID make provisions for makeup air right? If the 480CFM is for real I wouldn't count on that much just leaking through the cracks in your house.

  9. #9
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    Quote Originally Posted by 54regcab View Post
    If the manufacturers data doesn't have fan curve figures the 480CFM could be total BS. Size/speed of blower wheel and horsepower of motor might get you close to knowing real CFM. Odd they use a 6" duct connector for such a high CFM.

    You DID make provisions for makeup air right? If the 480CFM is for real I wouldn't count on that much just leaking through the cracks in your house.
    I'd have to tear it apart to figure out the size of the wheel. I calculate 1.6" loss using 6" round or 0.83" with 7" and 0.47" with 8". Does that sound right to you? I think I'm just going to use 8" just to be safe.

    As for the makeup air of course I made provisions for it: opening the front door

  10. #10
    Join Date
    Feb 2003
    Location
    garland, texas
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    Make up air? Whats that? We will be installing a 1200 cfm residential kitchen hood later this month. We asked the sellers hood specialist about make up air. His response was the house is loose enough. So the bottom line is it's not about how it works but how it looks in kitchen.

  11. #11
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    Moore, Oklahoma, United States
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    Did you count your roof cap/louver? Some designs can add quite a bit of static pressure at 480CFM. Most roof caps for kitchen hoods are only designed for 200CFM. Does the range hood have a backdraft damper or will it be in the roof cap/louver? I'd go 8" to compensate for all the extra 90's.

  12. #12
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    Quote Originally Posted by nvr2old View Post
    Make up air? Whats that? We will be installing a 1200 cfm residential kitchen hood later this month. We asked the sellers hood specialist about make up air. His response was the house is loose enough. So the bottom line is it's not about how it works but how it looks in kitchen.
    If you're the installing company, I'd highly recommend you check for any combustion appliances in the home and have the place tested by a HERS or BPI certified consultant before the home is released for occupancy. 1200 CFM exhaust with no make-up air source if a recipe for carbon monoxide poisoning if there's a gas or oil heating system in the home where the combustion process is not sealed. Your liability is out there for as long as that hood is installed, until someone else replaces it.

  13. #13
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    asdflkj

    If the fan company isn't publishing good info like REAL EXHAUST manufacturers then what con you do?. Is anyone really going to care if you only get 400 CFMs? Where is the make-up coming from?

    I appreciate your wanting to do things right but you can only do what you have good data for. Think the manufacturer has as much interest as you do?

    I would think about the real probability ( not possibility) of what happens when that duct begins to accumulate grease. Is there going to be access and cleanout possibilities? If not then someone could come knocking on your door when the house burns down and want to know who your insurance carrier was at the time, and ddi you have design insurance!
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

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