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  1. #14
    Join Date
    Jul 2005
    Posts
    126
    Originally posted by lginla
    Most of the replies here imply that a duct that is "too big" is bad...

    Then how do you design a ductwork for a two-stage system whose air flow varies by 40% or so? If you size it for the higher stage, won't the system then be "oversized" for the low stage, which most respondents say is bad?
    Sounds like you already know the answer: oversized ducts are not a bad thing. Only a small hit for cost, and possibly some routing issues if you're running through a floor system, etc.

    Having said that, if the diffusers are too large, air throw may be less than optimal. However, this becomes a non-issue in a properly insulated house.

    Finally, balancing, which is indeed important, can be accomplished with an oversized duct system in the same way as with a 'properly' sized duct system (e.g., by selecting correct relationships between various trunks, run-outs and difussers, and if necessary, using balancing dampers). See my earlier response, above.

    David

  2. #15
    Join Date
    Aug 2005
    Posts
    52
    Originally posted by dash
    Now the two speed is most often used with a variable speed fan,so cfm is being maintained.
    I assume you mean two-stage, not two-speed? But from what I see in manufacturers' brochures, they do not maintain constant CFM between first and second stages. They maintain a constant temperature rise, but the CFM varies by around 40% to match the increased heat output.

    So I still wonder how the other respondents who say that "oversized" ducts are bad because there is not enough backpressure or the duct velocity will be too low approach two-stage systems? You can't go in and manually adjust dampers as the unit cycles between first and second stage, can you?

  3. #16
    Join Date
    Aug 2005
    Posts
    52
    Originally posted by ginahoy


    Sounds like you already know the answer: oversized ducts are not a bad thing. Only a small hit for cost, and possibly some routing issues if you're running through a floor system, etc.
    David,

    Thanks, and I want to agree with you , but most of the other respondents said that "oversized" ducts are in fact bad. So I am curious how they size ducts for two stage systems, with the variable air flow.

  4. #17
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Well,Carrier calls theirs a two speed,but yes it's really two stage.


    The constant cfm I'm refering to,is that a Variable speed delivers the cfm required,be it 80 or 60% of high speed over a wide range of external Static Pressures.A PSC motor doesn't do that,as the static is reduced it moves more air,if still on the same speed.

    So problems ,if any ,due to oversized ducts, would be more likely with a PSC motor.


  5. #18
    Join Date
    Aug 2005
    Posts
    52
    dash,

    I understand that the VS will maintain the design cfm much better than a PSC.

    But even with a VS, it will have a significantly different cfm between low stage and high stage, according to manufacturers' specs.

    So how does one design the duct system (for the high stage, presumably), without having it become "oversized" for the low stage and its reduced air flow?

  6. #19
    Join Date
    Apr 2004
    Location
    Bartlett, IL
    Posts
    6,619
    How "oversized" are you talkin about? A couple inches or feet? Oversizing a little won't hurt the system. On Two stage systems, 1st stage is 65% this is why it distributes the air slowly & evenly in most cases.

  7. #20
    Join Date
    Aug 2005
    Posts
    52
    Originally posted by 2hot2coolme
    How "oversized" are you talkin about? A couple inches or feet? Oversizing a little won't hurt the system. On Two stage systems, 1st stage is 65% this is why it distributes the air slowly & evenly in most cases.
    The original poster mentioned oversizing by about 30%, which happens to be pretty close to the CFM difference between first and second stages of most units. And yet he got replies saying that this would be inefficient and grossly oversized.

    But how else would one design the duct system? If it sized "just right" for the second stage (high CFM), it will be automatically around 30% oversized for the first stage (low CFM). See my question?


  8. #21
    Join Date
    Apr 2004
    Location
    Bartlett, IL
    Posts
    6,619
    Well, I can't say I've come across too many "oversized" duct systems, Mainly "undersized". Now if the main trunkline is oversized, being one size without any change fittings, then the air will get lost into the duct, end result, your system will run like ****!

  9. #22
    Join Date
    Aug 2005
    Posts
    52
    Originally posted by 2hot2coolme
    Now if the main trunkline is oversized, being one size without any change fittings, then the air will get lost into the duct, end result, your system will run like ****!
    Well, but if the trunkline is sized "right" for the high stage CFM, it WILL become "oversized" for the low stage CFM, because you are moving less air through it.

    Does that mean that systems sized properly for high stage will run like sh*t on low stage? Doesn't sound right to me.

  10. #23
    Join Date
    Apr 2004
    Location
    Bartlett, IL
    Posts
    6,619
    Your not getting it! No it's not gonna run like **** on first stage if the system is sized right for second. It will run just fine.

  11. #24
    Join Date
    Aug 2005
    Posts
    52
    I hear what you are saying - if the system is sized right for the high stage, it'll be just fine for the low stage.

    I was just wondering why the other respondents thought that a 30% oversize will cause the "air to get lost", because that is exactly what happens when a unit runs on low stage through ductwork sized for high stage.

  12. #25
    Join Date
    Mar 2004
    Location
    CHICAGO SUBURBS
    Posts
    464
    In the new construction around where I live and work, you will definately not find any oversized duct work. They undersize so badly sometimes that a bypass humidifier will open the hi temp CU due to high supply temp giong right back into an undersized return. People buy the 3M filters and the HTCU opens. You need to use a piece of chicken wire for a filter to get close to proper flows {kidding but not far off}.Builders try to tell me how big of a dead space I can have to run duct.I say thanks but no thanks and Joe Hack does what they ask. I know I swayed a little of thread.
    Some people swear by me and some at me

  13. #26
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by lginla
    dash,

    I understand that the VS will maintain the design cfm much better than a PSC.

    But even with a VS, it will have a significantly different cfm between low stage and high stage, according to manufacturers' specs.

    So how does one design the duct system (for the high stage, presumably), without having it become "oversized" for the low stage and its reduced air flow?

    Somehow what I'm saying is not clear.

    We have hundreds of Carrier two speeds/stages,installed and there is NO PROBLEM.they work like a charm,"oversized" ducts "hehe!",and all.

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